path: root/src/VBox/Runtime/generic/semrw-generic.cpp

/* $Id: semrw-generic.cpp $ */
/** @file
 * IPRT - Read-Write Semaphore, Generic.
 *
 * This is a generic implementation for OSes which don't have
 * native RW semaphores.
 */

/*
 * Copyright (C) 2006-2023 Oracle and/or its affiliates.
 *
 * This file is part of VirtualBox base platform packages, as
 * available from https://www.virtualbox.org.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation, in version 3 of the
 * License.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <https://www.gnu.org/licenses>.
 *
 * The contents of this file may alternatively be used under the terms
 * of the Common Development and Distribution License Version 1.0
 * (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
 * in the VirtualBox distribution, in which case the provisions of the
 * CDDL are applicable instead of those of the GPL.
 *
 * You may elect to license modified versions of this file under the
 * terms and conditions of either the GPL or the CDDL or both.
 *
 * SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
 */


/*********************************************************************************************************************************
*   Header Files                                                                                                                 *
*********************************************************************************************************************************/
#define RTSEMRW_WITHOUT_REMAPPING
#include <iprt/semaphore.h>
#include "internal/iprt.h"

#include <iprt/asm.h>
#include <iprt/assert.h>
#include <iprt/critsect.h>
#include <iprt/err.h>
#include <iprt/lockvalidator.h>
#include <iprt/mem.h>
#include <iprt/time.h>
#include <iprt/thread.h>

#include "internal/magics.h"
#include "internal/strict.h"


/*********************************************************************************************************************************
*   Structures and Typedefs                                                                                                      *
*********************************************************************************************************************************/

/** Internal representation of a Read-Write semaphore for the
 * Generic implementation. */
struct RTSEMRWINTERNAL
{
    /** The usual magic. (RTSEMRW_MAGIC) */
    uint32_t            u32Magic;
    /* Alignment padding. */
    uint32_t            u32Padding;
    /** This critical section serializes the access to and updating of the structure members. */
    RTCRITSECT          CritSect;
    /** The current number of reads. (pure read recursion counts too) */
    uint32_t            cReads;
    /** The current number of writes. (recursion counts too) */
    uint32_t            cWrites;
    /** Number of read recursions by the writer. */
    uint32_t            cWriterReads;
    /** Number of writers waiting. */
    uint32_t            cWritesWaiting;
    /** The write owner of the lock. */
    RTNATIVETHREAD      hWriter;
    /** The handle of the event object on which the waiting readers block. (manual reset). */
    RTSEMEVENTMULTI     ReadEvent;
    /** The handle of the event object on which the waiting writers block. (automatic reset). */
    RTSEMEVENT          WriteEvent;
    /** Need to reset ReadEvent. */
    bool                fNeedResetReadEvent;
#ifdef RTSEMRW_STRICT
    /** The validator record for the writer. */
    RTLOCKVALRECEXCL    ValidatorWrite;
    /** The validator record for the readers. */
    RTLOCKVALRECSHRD    ValidatorRead;
#endif
};



RTDECL(int) RTSemRWCreate(PRTSEMRW phRWSem)
{
    return RTSemRWCreateEx(phRWSem, 0 /*fFlags*/, NIL_RTLOCKVALCLASS, RTLOCKVAL_SUB_CLASS_NONE, "RTSemRW");
}
RT_EXPORT_SYMBOL(RTSemRWCreate);


RTDECL(int) RTSemRWCreateEx(PRTSEMRW phRWSem, uint32_t fFlags,
                            RTLOCKVALCLASS hClass, uint32_t uSubClass, const char *pszNameFmt, ...)
{
    AssertReturn(!(fFlags & ~RTSEMRW_FLAGS_NO_LOCK_VAL), VERR_INVALID_PARAMETER);

    /*
     * Allocate memory.
     */
    int rc;
    struct RTSEMRWINTERNAL *pThis = (struct RTSEMRWINTERNAL *)RTMemAlloc(sizeof(struct RTSEMRWINTERNAL));
    if (pThis)
    {
        /*
         * Create the semaphores.
         */
        rc = RTSemEventCreateEx(&pThis->WriteEvent, RTSEMEVENT_FLAGS_NO_LOCK_VAL, NIL_RTLOCKVALCLASS, NULL);
        if (RT_SUCCESS(rc))
        {
            rc = RTSemEventMultiCreateEx(&pThis->ReadEvent, RTSEMEVENT_FLAGS_NO_LOCK_VAL, NIL_RTLOCKVALCLASS, NULL);
            if (RT_SUCCESS(rc))
            {
                rc = RTCritSectInitEx(&pThis->CritSect, RTCRITSECT_FLAGS_NO_LOCK_VAL,
                                      NIL_RTLOCKVALCLASS, RTLOCKVAL_SUB_CLASS_NONE, NULL);
                if (RT_SUCCESS(rc))
                {
                    /*
                     * Signal the read semaphore and initialize other variables.
                     */
                    rc = RTSemEventMultiSignal(pThis->ReadEvent);
                    if (RT_SUCCESS(rc))
                    {
                        pThis->u32Padding           = UINT32_C(0xa5a55a5a);
                        pThis->cReads               = 0;
                        pThis->cWrites              = 0;
                        pThis->cWriterReads         = 0;
                        pThis->cWritesWaiting       = 0;
                        pThis->hWriter              = NIL_RTNATIVETHREAD;
                        pThis->fNeedResetReadEvent  = true;
                        pThis->u32Magic             = RTSEMRW_MAGIC;
#ifdef RTSEMRW_STRICT
                        bool const fLVEnabled = !(fFlags & RTSEMRW_FLAGS_NO_LOCK_VAL);
                        if (!pszNameFmt)
                        {
                            static uint32_t volatile s_iSemRWAnon = 0;
                            uint32_t i = ASMAtomicIncU32(&s_iSemRWAnon) - 1;
                            RTLockValidatorRecExclInit(&pThis->ValidatorWrite, hClass, uSubClass, pThis,
                                                       fLVEnabled, "RTSemRW-%u", i);
                            RTLockValidatorRecSharedInit(&pThis->ValidatorRead, hClass, uSubClass, pThis,
                                                         false /*fSignaller*/, fLVEnabled, "RTSemRW-%u", i);
                        }
                        else
                        {
                            va_list va;
                            va_start(va, pszNameFmt);
                            RTLockValidatorRecExclInitV(&pThis->ValidatorWrite, hClass, uSubClass, pThis,
                                                        fLVEnabled, pszNameFmt, va);
                            va_end(va);
                            va_start(va, pszNameFmt);
                            RTLockValidatorRecSharedInitV(&pThis->ValidatorRead, hClass, uSubClass, pThis,
                                                          false /*fSignaller*/, fLVEnabled, pszNameFmt, va);
                            va_end(va);
                        }
                        RTLockValidatorRecMakeSiblings(&pThis->ValidatorWrite.Core, &pThis->ValidatorRead.Core);
#else
                        RT_NOREF_PV(hClass); RT_NOREF_PV(uSubClass); RT_NOREF_PV(pszNameFmt);
#endif
                        *phRWSem = pThis;
                        return VINF_SUCCESS;
                    }
                    RTCritSectDelete(&pThis->CritSect);
                }
                RTSemEventMultiDestroy(pThis->ReadEvent);
            }
            RTSemEventDestroy(pThis->WriteEvent);
        }
        RTMemFree(pThis);
    }
    else
        rc = VERR_NO_MEMORY;

    return rc;
}
RT_EXPORT_SYMBOL(RTSemRWCreate);


RTDECL(int) RTSemRWDestroy(RTSEMRW hRWSem)
{
    struct RTSEMRWINTERNAL *pThis = hRWSem;

    /*
     * Validate handle.
     */
    if (pThis == NIL_RTSEMRW)
        return VINF_SUCCESS;
    AssertPtrReturn(pThis, VERR_INVALID_HANDLE);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, VERR_INVALID_HANDLE);

    /*
     * Check if busy.
     */
    int rc = RTCritSectTryEnter(&pThis->CritSect);
    if (RT_SUCCESS(rc))
    {
        if (!pThis->cReads && !pThis->cWrites)
        {
            /*
             * Make it invalid and unusable.
             */
            ASMAtomicWriteU32(&pThis->u32Magic, ~RTSEMRW_MAGIC);
            pThis->cReads = UINT32_MAX;

            /*
             * Do actual cleanup. None of these can now fail.
             */
            rc = RTSemEventMultiDestroy(pThis->ReadEvent);
            AssertMsgRC(rc, ("RTSemEventMultiDestroy failed! rc=%Rrc\n", rc));
            pThis->ReadEvent = NIL_RTSEMEVENTMULTI;

            rc = RTSemEventDestroy(pThis->WriteEvent);
            AssertMsgRC(rc, ("RTSemEventDestroy failed! rc=%Rrc\n", rc));
            pThis->WriteEvent = NIL_RTSEMEVENT;

            RTCritSectLeave(&pThis->CritSect);
            rc = RTCritSectDelete(&pThis->CritSect);
            AssertMsgRC(rc, ("RTCritSectDelete failed! rc=%Rrc\n", rc));

#ifdef RTSEMRW_STRICT
            RTLockValidatorRecSharedDelete(&pThis->ValidatorRead);
            RTLockValidatorRecExclDelete(&pThis->ValidatorWrite);
#endif
            RTMemFree(pThis);
            rc = VINF_SUCCESS;
        }
        else
        {
            rc = VERR_SEM_BUSY;
            RTCritSectLeave(&pThis->CritSect);
        }
    }
    else
    {
        AssertMsgRC(rc, ("RTCritSectTryEnter failed! rc=%Rrc\n", rc));
        rc = VERR_SEM_BUSY;
    }

    return rc;
}
RT_EXPORT_SYMBOL(RTSemRWDestroy);


RTDECL(uint32_t) RTSemRWSetSubClass(RTSEMRW hRWSem, uint32_t uSubClass)
{
#ifdef RTSEMRW_STRICT
    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, RTLOCKVAL_SUB_CLASS_INVALID);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, RTLOCKVAL_SUB_CLASS_INVALID);

    RTLockValidatorRecSharedSetSubClass(&pThis->ValidatorRead, uSubClass);
    return RTLockValidatorRecExclSetSubClass(&pThis->ValidatorWrite, uSubClass);
#else
    RT_NOREF_PV(hRWSem); RT_NOREF_PV(uSubClass);
    return RTLOCKVAL_SUB_CLASS_INVALID;
#endif
}
RT_EXPORT_SYMBOL(RTSemRWSetSubClass);


DECL_FORCE_INLINE(int) rtSemRWRequestRead(RTSEMRW hRWSem, RTMSINTERVAL cMillies, bool fInterruptible, PCRTLOCKVALSRCPOS pSrcPos)
{
    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, VERR_INVALID_HANDLE);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, VERR_INVALID_HANDLE);

    RTMSINTERVAL    cMilliesInitial = cMillies;
    uint64_t        tsStart = 0;
    if (cMillies != RT_INDEFINITE_WAIT && cMillies != 0)
        tsStart = RTTimeNanoTS();

#ifdef RTSEMRW_STRICT
    RTTHREAD hThreadSelf = RTThreadSelfAutoAdopt();
    if (cMillies > 0)
    {
        int rc9;
        if (pThis->hWriter != NIL_RTTHREAD && pThis->hWriter == RTThreadNativeSelf())
            rc9 = RTLockValidatorRecExclCheckOrder(&pThis->ValidatorWrite, hThreadSelf, pSrcPos, cMillies);
        else
            rc9 = RTLockValidatorRecSharedCheckOrder(&pThis->ValidatorRead, hThreadSelf, pSrcPos, cMillies);
        if (RT_FAILURE(rc9))
            return rc9;
    }
#endif

    /*
     * Take critsect.
     */
    int rc = RTCritSectEnter(&pThis->CritSect);
    if (RT_FAILURE(rc))
    {
        AssertMsgFailed(("RTCritSectEnter failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
        return rc;
    }

    /*
     * Check if the state of affairs allows read access.
     * Do not block further readers if there is a writer waiting, as
     * that will break/deadlock reader recursion.
     */
    if (    pThis->hWriter == NIL_RTNATIVETHREAD
#if 0
        && (   !pThis->cWritesWaiting
            ||  pThis->cReads)
#endif
       )
    {
        pThis->cReads++;
        Assert(pThis->cReads > 0);
#ifdef RTSEMRW_STRICT
        RTLockValidatorRecSharedAddOwner(&pThis->ValidatorRead, hThreadSelf, pSrcPos);
#endif

        RTCritSectLeave(&pThis->CritSect);
        return VINF_SUCCESS;
    }

    RTNATIVETHREAD hNativeSelf = pThis->CritSect.NativeThreadOwner;
    if (pThis->hWriter == hNativeSelf)
    {
#ifdef RTSEMRW_STRICT
        int rc9 = RTLockValidatorRecExclRecursionMixed(&pThis->ValidatorWrite, &pThis->ValidatorRead.Core, pSrcPos);
        if (RT_FAILURE(rc9))
        {
            RTCritSectLeave(&pThis->CritSect);
            return rc9;
        }
#endif

        pThis->cWriterReads++;
        Assert(pThis->cWriterReads > 0);

        RTCritSectLeave(&pThis->CritSect);
        return VINF_SUCCESS;
    }

    RTCritSectLeave(&pThis->CritSect);

    /*
     * Wait till it's ready for reading.
     */
    if (cMillies == 0)
        return VERR_TIMEOUT;

#ifndef RTSEMRW_STRICT
    RTTHREAD hThreadSelf = RTThreadSelf();
#endif
    for (;;)
    {
        if (cMillies != RT_INDEFINITE_WAIT)
        {
            int64_t tsDelta = RTTimeNanoTS() - tsStart;
            if (tsDelta >= 1000000)
            {
                tsDelta /= 1000000;
                if ((uint64_t)tsDelta < cMilliesInitial)
                    cMilliesInitial = (RTMSINTERVAL)tsDelta;
                else
                    cMilliesInitial = 1;
            }
        }
#ifdef RTSEMRW_STRICT
        rc = RTLockValidatorRecSharedCheckBlocking(&pThis->ValidatorRead, hThreadSelf, pSrcPos, true,
                                                   cMillies, RTTHREADSTATE_RW_READ, false);
        if (RT_FAILURE(rc))
            break;
#else
        RTThreadBlocking(hThreadSelf, RTTHREADSTATE_RW_READ, false);
        RT_NOREF_PV(pSrcPos);
#endif
        int rcWait;
        if (fInterruptible)
            rcWait = rc = RTSemEventMultiWaitNoResume(pThis->ReadEvent, cMillies);
        else
            rcWait = rc = RTSemEventMultiWait(pThis->ReadEvent, cMillies);
        RTThreadUnblocked(hThreadSelf, RTTHREADSTATE_RW_READ);
        if (RT_FAILURE(rc) && rc != VERR_TIMEOUT) /* handle timeout below */
        {
            AssertMsgRC(rc, ("RTSemEventMultiWait failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
            break;
        }

        if (pThis->u32Magic != RTSEMRW_MAGIC)
        {
            rc = VERR_SEM_DESTROYED;
            break;
        }

        /*
         * Re-take critsect and repeat the check we did before the loop.
         */
        rc = RTCritSectEnter(&pThis->CritSect);
        if (RT_FAILURE(rc))
        {
            AssertMsgFailed(("RTCritSectEnter failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
            break;
        }

        if (    pThis->hWriter == NIL_RTNATIVETHREAD
#if 0
            && (   !pThis->cWritesWaiting
                ||  pThis->cReads)
#endif
           )
        {
            pThis->cReads++;
            Assert(pThis->cReads > 0);
#ifdef RTSEMRW_STRICT
            RTLockValidatorRecSharedAddOwner(&pThis->ValidatorRead, hThreadSelf, pSrcPos);
#endif

            RTCritSectLeave(&pThis->CritSect);
            return VINF_SUCCESS;
        }

        RTCritSectLeave(&pThis->CritSect);

        /*
         * Quit if the wait already timed out.
         */
        if (rcWait == VERR_TIMEOUT)
        {
            rc = VERR_TIMEOUT;
            break;
        }
    }

    /* failed */
    return rc;
}


RTDECL(int) RTSemRWRequestRead(RTSEMRW hRWSem, RTMSINTERVAL cMillies)
{
#ifndef RTSEMRW_STRICT
    return rtSemRWRequestRead(hRWSem, cMillies, false, NULL);
#else
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_NORMAL_API();
    return rtSemRWRequestRead(hRWSem, cMillies, false, &SrcPos);
#endif
}
RT_EXPORT_SYMBOL(RTSemRWRequestRead);


RTDECL(int) RTSemRWRequestReadDebug(RTSEMRW hRWSem, RTMSINTERVAL cMillies, RTHCUINTPTR uId, RT_SRC_POS_DECL)
{
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_DEBUG_API();
    return rtSemRWRequestRead(hRWSem, cMillies, false, &SrcPos);
}
RT_EXPORT_SYMBOL(RTSemRWRequestReadDebug);


RTDECL(int) RTSemRWRequestReadNoResume(RTSEMRW hRWSem, RTMSINTERVAL cMillies)
{
#ifndef RTSEMRW_STRICT
    return rtSemRWRequestRead(hRWSem, cMillies, true, NULL);
#else
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_NORMAL_API();
    return rtSemRWRequestRead(hRWSem, cMillies, true, &SrcPos);
#endif
}
RT_EXPORT_SYMBOL(RTSemRWRequestReadNoResume);


RTDECL(int) RTSemRWRequestReadNoResumeDebug(RTSEMRW hRWSem, RTMSINTERVAL cMillies, RTHCUINTPTR uId, RT_SRC_POS_DECL)
{
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_DEBUG_API();
    return rtSemRWRequestRead(hRWSem, cMillies, true, &SrcPos);
}
RT_EXPORT_SYMBOL(RTSemRWRequestReadNoResumeDebug);


RTDECL(int) RTSemRWReleaseRead(RTSEMRW hRWSem)
{
    struct RTSEMRWINTERNAL *pThis = hRWSem;

    /*
     * Validate handle.
     */
    AssertPtrReturn(pThis, VERR_INVALID_HANDLE);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, VERR_INVALID_HANDLE);

    /*
     * Take critsect.
     */
    int rc = RTCritSectEnter(&pThis->CritSect);
    if (RT_SUCCESS(rc))
    {
        if (pThis->hWriter == NIL_RTNATIVETHREAD)
        {
#ifdef RTSEMRW_STRICT
            rc = RTLockValidatorRecSharedCheckAndRelease(&pThis->ValidatorRead, NIL_RTTHREAD);
            if (RT_SUCCESS(rc))
#endif
            {
                if (RT_LIKELY(pThis->cReads > 0))
                {
                    pThis->cReads--;

                    /* Kick off a writer if appropriate. */
                    if (    pThis->cWritesWaiting > 0
                        &&  !pThis->cReads)
                    {
                        rc = RTSemEventSignal(pThis->WriteEvent);
                        AssertMsgRC(rc, ("Failed to signal writers on rwsem %p, rc=%Rrc\n", hRWSem, rc));
                    }
                }
                else
                {
                    AssertFailed();
                    rc = VERR_NOT_OWNER;
                }
            }
        }
        else
        {
            RTNATIVETHREAD hNativeSelf = pThis->CritSect.NativeThreadOwner;
            if (pThis->hWriter == hNativeSelf)
            {
                if (pThis->cWriterReads > 0)
                {
#ifdef RTSEMRW_STRICT
                    rc = RTLockValidatorRecExclUnwindMixed(&pThis->ValidatorWrite, &pThis->ValidatorRead.Core);
                    if (RT_SUCCESS(rc))
#endif
                    {
                        pThis->cWriterReads--;
                    }
                }
                else
                {
                    AssertFailed();
                    rc = VERR_NOT_OWNER;
                }
            }
            else
            {
                AssertFailed();
                rc = VERR_NOT_OWNER;
            }
        }

        RTCritSectLeave(&pThis->CritSect);
    }
    else
        AssertMsgFailed(("RTCritSectEnter failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));

    return rc;
}
RT_EXPORT_SYMBOL(RTSemRWReleaseRead);


DECL_FORCE_INLINE(int) rtSemRWRequestWrite(RTSEMRW hRWSem, RTMSINTERVAL cMillies, bool fInterruptible, PCRTLOCKVALSRCPOS pSrcPos)
{
    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis   = hRWSem;
    AssertPtrReturn(pThis, VERR_INVALID_HANDLE);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, VERR_INVALID_HANDLE);

    RTMSINTERVAL    cMilliesInitial = cMillies;
    uint64_t        tsStart         = 0;
    if (cMillies != RT_INDEFINITE_WAIT && cMillies != 0)
        tsStart = RTTimeNanoTS();

#ifdef RTSEMRW_STRICT
    RTTHREAD hThreadSelf = NIL_RTTHREAD;
    if (cMillies)
    {
        hThreadSelf = RTThreadSelfAutoAdopt();
        int rc9 = RTLockValidatorRecExclCheckOrder(&pThis->ValidatorWrite, hThreadSelf, pSrcPos, cMillies);
        if (RT_FAILURE(rc9))
            return rc9;
    }
#endif

    /*
     * Take critsect.
     */
    int rc = RTCritSectEnter(&pThis->CritSect);
    if (RT_FAILURE(rc))
    {
        AssertMsgFailed(("RTCritSectEnter failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
        return rc;
    }

    /*
     * Check if the state of affairs allows write access.
     */
    RTNATIVETHREAD hNativeSelf = pThis->CritSect.NativeThreadOwner;
    if (    !pThis->cReads
        &&  (   (   !pThis->cWrites
                 && (   !pThis->cWritesWaiting /* play fair if we can wait */
                     || !cMillies)
                )
             || pThis->hWriter == hNativeSelf
            )
       )
    {
        /*
         * Reset the reader event semaphore if necessary.
         */
        if (pThis->fNeedResetReadEvent)
        {
            pThis->fNeedResetReadEvent = false;
            rc = RTSemEventMultiReset(pThis->ReadEvent);
            AssertMsgRC(rc, ("Failed to reset readers, rwsem %p, rc=%Rrc.\n", hRWSem, rc));
        }

        pThis->cWrites++;
        pThis->hWriter = hNativeSelf;
#ifdef RTSEMRW_STRICT
        RTLockValidatorRecExclSetOwner(&pThis->ValidatorWrite, hThreadSelf, pSrcPos, pThis->cWrites == 1);
#endif
        RTCritSectLeave(&pThis->CritSect);
        return VINF_SUCCESS;
    }

    /*
     * Signal writer presence.
     */
    if (cMillies != 0)
        pThis->cWritesWaiting++;

    RTCritSectLeave(&pThis->CritSect);

    /*
     * Wait till it's ready for writing.
     */
    if (cMillies == 0)
        return VERR_TIMEOUT;

#ifndef RTSEMRW_STRICT
    RTTHREAD hThreadSelf = RTThreadSelf();
#endif
    for (;;)
    {
        if (cMillies != RT_INDEFINITE_WAIT)
        {
            int64_t tsDelta = RTTimeNanoTS() - tsStart;
            if (tsDelta >= 1000000)
            {
                tsDelta /= 1000000;
                if ((uint64_t)tsDelta < cMilliesInitial)
                    cMilliesInitial = (RTMSINTERVAL)tsDelta;
                else
                    cMilliesInitial = 1;
            }
        }

#ifdef RTSEMRW_STRICT
        rc = RTLockValidatorRecExclCheckBlocking(&pThis->ValidatorWrite, hThreadSelf, pSrcPos, true,
                                                 cMillies, RTTHREADSTATE_RW_WRITE, false);
        if (RT_FAILURE(rc))
            break;
#else
        RTThreadBlocking(hThreadSelf, RTTHREADSTATE_RW_WRITE, false);
        RT_NOREF_PV(pSrcPos);
#endif
        int rcWait;
        if (fInterruptible)
            rcWait = rc = RTSemEventWaitNoResume(pThis->WriteEvent, cMillies);
        else
            rcWait = rc = RTSemEventWait(pThis->WriteEvent, cMillies);
        RTThreadUnblocked(hThreadSelf, RTTHREADSTATE_RW_WRITE);
        if (RT_UNLIKELY(RT_FAILURE_NP(rc) && rc != VERR_TIMEOUT)) /* timeouts are handled below */
        {
            AssertMsgRC(rc, ("RTSemEventWait failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
            break;
        }

        if (RT_UNLIKELY(pThis->u32Magic != RTSEMRW_MAGIC))
        {
            rc = VERR_SEM_DESTROYED;
            break;
        }

        /*
         * Re-take critsect and repeat the check we did prior to this loop.
         */
        rc = RTCritSectEnter(&pThis->CritSect);
        if (RT_FAILURE(rc))
        {
            AssertMsgFailed(("RTCritSectEnter failed on rwsem %p, rc=%Rrc\n", hRWSem, rc));
            break;
        }

        if (!pThis->cReads && (!pThis->cWrites || pThis->hWriter == hNativeSelf))
        {
            /*
             * Reset the reader event semaphore if necessary.
             */
            if (pThis->fNeedResetReadEvent)
            {
                pThis->fNeedResetReadEvent = false;
                rc = RTSemEventMultiReset(pThis->ReadEvent);
                AssertMsgRC(rc, ("Failed to reset readers, rwsem %p, rc=%Rrc.\n", hRWSem, rc));
            }

            pThis->cWrites++;
            pThis->hWriter = hNativeSelf;
            pThis->cWritesWaiting--;
#ifdef RTSEMRW_STRICT
            RTLockValidatorRecExclSetOwner(&pThis->ValidatorWrite, hThreadSelf, pSrcPos, true);
#endif

            RTCritSectLeave(&pThis->CritSect);
            return VINF_SUCCESS;
        }

        RTCritSectLeave(&pThis->CritSect);

        /*
         * Quit if the wait already timed out.
         */
        if (rcWait == VERR_TIMEOUT)
        {
            rc = VERR_TIMEOUT;
            break;
        }
    }

    /*
     * Timeout/error case, clean up.
     */
    if (pThis->u32Magic == RTSEMRW_MAGIC)
    {
        RTCritSectEnter(&pThis->CritSect);
        /* Adjust this counter, whether we got the critsect or not. */
        pThis->cWritesWaiting--;
        RTCritSectLeave(&pThis->CritSect);
    }
    return rc;
}


RTDECL(int) RTSemRWRequestWrite(RTSEMRW hRWSem, RTMSINTERVAL cMillies)
{
#ifndef RTSEMRW_STRICT
    return rtSemRWRequestWrite(hRWSem, cMillies, false, NULL);
#else
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_NORMAL_API();
    return rtSemRWRequestWrite(hRWSem, cMillies, false, &SrcPos);
#endif
}
RT_EXPORT_SYMBOL(RTSemRWRequestWrite);


RTDECL(int) RTSemRWRequestWriteDebug(RTSEMRW hRWSem, RTMSINTERVAL cMillies, RTHCUINTPTR uId, RT_SRC_POS_DECL)
{
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_DEBUG_API();
    return rtSemRWRequestWrite(hRWSem, cMillies, false, &SrcPos);
}
RT_EXPORT_SYMBOL(RTSemRWRequestWriteDebug);


RTDECL(int) RTSemRWRequestWriteNoResume(RTSEMRW hRWSem, RTMSINTERVAL cMillies)
{
#ifndef RTSEMRW_STRICT
    return rtSemRWRequestWrite(hRWSem, cMillies, true, NULL);
#else
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_NORMAL_API();
    return rtSemRWRequestWrite(hRWSem, cMillies, true, &SrcPos);
#endif
}
RT_EXPORT_SYMBOL(RTSemRWRequestWriteNoResume);


RTDECL(int) RTSemRWRequestWriteNoResumeDebug(RTSEMRW hRWSem, RTMSINTERVAL cMillies, RTHCUINTPTR uId, RT_SRC_POS_DECL)
{
    RTLOCKVALSRCPOS SrcPos = RTLOCKVALSRCPOS_INIT_DEBUG_API();
    return rtSemRWRequestWrite(hRWSem, cMillies, true, &SrcPos);
}
RT_EXPORT_SYMBOL(RTSemRWRequestWriteNoResumeDebug);


RTDECL(int) RTSemRWReleaseWrite(RTSEMRW hRWSem)
{

    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, VERR_INVALID_HANDLE);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, VERR_INVALID_HANDLE);

    /*
     * Take critsect.
     */
    int rc = RTCritSectEnter(&pThis->CritSect);
    AssertRCReturn(rc, rc);

    /*
     * Check if owner.
     */
    RTNATIVETHREAD hNativeSelf = pThis->CritSect.NativeThreadOwner;
    if (pThis->hWriter != hNativeSelf)
    {
        RTCritSectLeave(&pThis->CritSect);
        AssertMsgFailed(("Not read-write owner of rwsem %p.\n", hRWSem));
        return VERR_NOT_OWNER;
    }

#ifdef RTSEMRW_STRICT
    if (pThis->cWrites > 1 || !pThis->cWriterReads) /* don't check+release if VERR_WRONG_ORDER */
    {
        int rc9 = RTLockValidatorRecExclReleaseOwner(&pThis->ValidatorWrite, pThis->cWrites == 1);
        if (RT_FAILURE(rc9))
        {
            RTCritSectLeave(&pThis->CritSect);
            return rc9;
        }
    }
#endif

    /*
     * Release ownership and remove ourselves from the writers count.
     */
    Assert(pThis->cWrites > 0);
    pThis->cWrites--;
    if (!pThis->cWrites)
    {
        if (RT_UNLIKELY(pThis->cWriterReads > 0))
        {
            pThis->cWrites++;
            RTCritSectLeave(&pThis->CritSect);
            AssertMsgFailed(("All recursive read locks need to be released prior to the final write lock! (%p)n\n", pThis));
            return VERR_WRONG_ORDER;
        }

        pThis->hWriter = NIL_RTNATIVETHREAD;
    }

    /*
     * Release the readers if no more writers waiting, otherwise the writers.
     */
    if (!pThis->cWritesWaiting)
    {
        rc = RTSemEventMultiSignal(pThis->ReadEvent);
        AssertMsgRC(rc, ("RTSemEventMultiSignal failed for rwsem %p, rc=%Rrc.\n", hRWSem, rc));
        pThis->fNeedResetReadEvent = true;
    }
    else
    {
        rc = RTSemEventSignal(pThis->WriteEvent);
        AssertMsgRC(rc, ("Failed to signal writers on rwsem %p, rc=%Rrc\n", hRWSem, rc));
    }
    RTCritSectLeave(&pThis->CritSect);

    return rc;
}
RT_EXPORT_SYMBOL(RTSemRWReleaseWrite);


RTDECL(bool) RTSemRWIsWriteOwner(RTSEMRW hRWSem)
{
    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, false);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, false);

    /*
     * Check ownership.
     */
    RTNATIVETHREAD hNativeSelf = RTThreadNativeSelf();
    RTNATIVETHREAD hWriter;
    ASMAtomicUoReadHandle(&pThis->hWriter, &hWriter);
    return hWriter == hNativeSelf;
}
RT_EXPORT_SYMBOL(RTSemRWIsWriteOwner);


RTDECL(bool)  RTSemRWIsReadOwner(RTSEMRW hRWSem, bool fWannaHear)
{
    /*
     * Validate handle.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, false);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, false);

    /*
     * Check write ownership.  The writer is also a valid reader.
     */
    RTNATIVETHREAD hNativeSelf = RTThreadNativeSelf();
    RTNATIVETHREAD hWriter;
    ASMAtomicUoReadHandle(&pThis->hWriter, &hWriter);
    if (hWriter == hNativeSelf)
        return true;
    if (hWriter != NIL_RTNATIVETHREAD)
        return false;

#ifdef RTSEMRW_STRICT
    /*
     * Ask the lock validator.
     */
    NOREF(fWannaHear);
    return RTLockValidatorRecSharedIsOwner(&pThis->ValidatorRead, NIL_RTTHREAD);
#else
    /*
     * If there are no reads we cannot be one of them... But if there are we
     * cannot know and can only return what the caller want to hear.
     */
    if (pThis->cReads == 0)
        return false;
    return fWannaHear;
#endif
}
RT_EXPORT_SYMBOL(RTSemRWIsReadOwner);


RTDECL(uint32_t) RTSemRWGetWriteRecursion(RTSEMRW hRWSem)
{
    struct RTSEMRWINTERNAL *pThis = hRWSem;

    /*
     * Validate handle.
     */
    AssertPtrReturn(pThis, 0);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, 0);

    /*
     * Return the requested data.
     */
    return pThis->cWrites;
}
RT_EXPORT_SYMBOL(RTSemRWGetWriteRecursion);


RTDECL(uint32_t) RTSemRWGetWriterReadRecursion(RTSEMRW hRWSem)
{
    struct RTSEMRWINTERNAL *pThis = hRWSem;

    /*
     * Validate handle.
     */
    AssertPtrReturn(pThis, 0);
    AssertReturn(pThis->u32Magic == RTSEMRW_MAGIC, 0);

    /*
     * Return the requested data.
     */
    return pThis->cWriterReads;
}
RT_EXPORT_SYMBOL(RTSemRWGetWriterReadRecursion);


RTDECL(uint32_t) RTSemRWGetReadCount(RTSEMRW hRWSem)
{
    /*
     * Validate input.
     */
    struct RTSEMRWINTERNAL *pThis = hRWSem;
    AssertPtrReturn(pThis, 0);
    AssertMsgReturn(pThis->u32Magic == RTSEMRW_MAGIC,
                    ("pThis=%p u32Magic=%#x\n", pThis, pThis->u32Magic),
                    0);

    /*
     * Return the requested data.
     */
    return pThis->cReads;
}
RT_EXPORT_SYMBOL(RTSemRWGetReadCount);