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Diffstat (limited to 'Documentation/filesystems/files.rst')
| -rw-r--r-- | Documentation/filesystems/files.rst | 53 |
1 files changed, 24 insertions, 29 deletions
diff --git a/Documentation/filesystems/files.rst b/Documentation/filesystems/files.rst index bcf8445991..9e38e4c221 100644 --- a/Documentation/filesystems/files.rst +++ b/Documentation/filesystems/files.rst @@ -62,7 +62,7 @@ the fdtable structure - be held. 4. To look up the file structure given an fd, a reader - must use either lookup_fd_rcu() or files_lookup_fd_rcu() APIs. These + must use either lookup_fdget_rcu() or files_lookup_fdget_rcu() APIs. These take care of barrier requirements due to lock-free lookup. An example:: @@ -70,43 +70,22 @@ the fdtable structure - struct file *file; rcu_read_lock(); - file = lookup_fd_rcu(fd); - if (file) { - ... - } - .... + file = lookup_fdget_rcu(fd); rcu_read_unlock(); - -5. Handling of the file structures is special. Since the look-up - of the fd (fget()/fget_light()) are lock-free, it is possible - that look-up may race with the last put() operation on the - file structure. This is avoided using atomic_long_inc_not_zero() - on ->f_count:: - - rcu_read_lock(); - file = files_lookup_fd_rcu(files, fd); if (file) { - if (atomic_long_inc_not_zero(&file->f_count)) - *fput_needed = 1; - else - /* Didn't get the reference, someone's freed */ - file = NULL; + ... + fput(file); } - rcu_read_unlock(); .... - return file; - - atomic_long_inc_not_zero() detects if refcounts is already zero or - goes to zero during increment. If it does, we fail - fget()/fget_light(). -6. Since both fdtable and file structures can be looked up +5. Since both fdtable and file structures can be looked up lock-free, they must be installed using rcu_assign_pointer() API. If they are looked up lock-free, rcu_dereference() must be used. However it is advisable to use files_fdtable() - and lookup_fd_rcu()/files_lookup_fd_rcu() which take care of these issues. + and lookup_fdget_rcu()/files_lookup_fdget_rcu() which take care of these + issues. -7. While updating, the fdtable pointer must be looked up while +6. While updating, the fdtable pointer must be looked up while holding files->file_lock. If ->file_lock is dropped, then another thread expand the files thereby creating a new fdtable and making the earlier fdtable pointer stale. @@ -126,3 +105,19 @@ the fdtable structure - Since locate_fd() can drop ->file_lock (and reacquire ->file_lock), the fdtable pointer (fdt) must be loaded after locate_fd(). +On newer kernels rcu based file lookup has been switched to rely on +SLAB_TYPESAFE_BY_RCU instead of call_rcu(). It isn't sufficient anymore +to just acquire a reference to the file in question under rcu using +atomic_long_inc_not_zero() since the file might have already been +recycled and someone else might have bumped the reference. In other +words, callers might see reference count bumps from newer users. For +this is reason it is necessary to verify that the pointer is the same +before and after the reference count increment. This pattern can be seen +in get_file_rcu() and __files_get_rcu(). + +In addition, it isn't possible to access or check fields in struct file +without first aqcuiring a reference on it under rcu lookup. Not doing +that was always very dodgy and it was only usable for non-pointer data +in struct file. With SLAB_TYPESAFE_BY_RCU it is necessary that callers +either first acquire a reference or they must hold the files_lock of the +fdtable. |