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Diffstat (limited to 'drivers/block/paride/Transition-notes')
| -rw-r--r-- | drivers/block/paride/Transition-notes | 128 |
1 files changed, 128 insertions, 0 deletions
diff --git a/drivers/block/paride/Transition-notes b/drivers/block/paride/Transition-notes new file mode 100644 index 000000000..70374907c --- /dev/null +++ b/drivers/block/paride/Transition-notes @@ -0,0 +1,128 @@ +Lemma 1: + If ps_tq is scheduled, ps_tq_active is 1. ps_tq_int() can be called + only when ps_tq_active is 1. +Proof: All assignments to ps_tq_active and all scheduling of ps_tq happen + under ps_spinlock. There are three places where that can happen: + one in ps_set_intr() (A) and two in ps_tq_int() (B and C). + Consider the sequnce of these events. A can not be preceded by + anything except B, since it is under if (!ps_tq_active) under + ps_spinlock. C is always preceded by B, since we can't reach it + other than through B and we don't drop ps_spinlock between them. + IOW, the sequence is A?(BA|BC|B)*. OTOH, number of B can not exceed + the sum of numbers of A and C, since each call of ps_tq_int() is + the result of ps_tq execution. Therefore, the sequence starts with + A and each B is preceded by either A or C. Moments when we enter + ps_tq_int() are sandwiched between {A,C} and B in that sequence, + since at any time number of B can not exceed the number of these + moments which, in turn, can not exceed the number of A and C. + In other words, the sequence of events is (A or C set ps_tq_active to + 1 and schedule ps_tq, ps_tq is executed, ps_tq_int() is entered, + B resets ps_tq_active)*. + + +consider the following area: + * in do_pd_request1(): to calls of pi_do_claimed() and return in + case when pd_req is NULL. + * in next_request(): to call of do_pd_request1() + * in do_pd_read(): to call of ps_set_intr() + * in do_pd_read_start(): to calls of pi_do_claimed(), next_request() +and ps_set_intr() + * in do_pd_read_drq(): to calls of pi_do_claimed() and next_request() + * in do_pd_write(): to call of ps_set_intr() + * in do_pd_write_start(): to calls of pi_do_claimed(), next_request() +and ps_set_intr() + * in do_pd_write_done(): to calls of pi_do_claimed() and next_request() + * in ps_set_intr(): to check for ps_tq_active and to scheduling + ps_tq if ps_tq_active was 0. + * in ps_tq_int(): from the moment when we get ps_spinlock() to the + return, call of con() or scheduling ps_tq. + * in pi_schedule_claimed() when called from pi_do_claimed() called from + pd.c, everything until returning 1 or setting or setting ->claim_cont + on the path that returns 0 + * in pi_do_claimed() when called from pd.c, everything until the call + of pi_do_claimed() plus the everything until the call of cont() if + pi_do_claimed() has returned 1. + * in pi_wake_up() called for PIA that belongs to pd.c, everything from + the moment when pi_spinlock has been acquired. + +Lemma 2: + 1) at any time at most one thread of execution can be in that area or + be preempted there. + 2) When there is such a thread, pd_busy is set or pd_lock is held by + that thread. + 3) When there is such a thread, ps_tq_active is 0 or ps_spinlock is + held by that thread. + 4) When there is such a thread, all PIA belonging to pd.c have NULL + ->claim_cont or pi_spinlock is held by thread in question. + +Proof: consider the first moment when the above is not true. + +(1) can become not true if some thread enters that area while another is there. + a) do_pd_request1() can be called from next_request() or do_pd_request() + In the first case the thread was already in the area. In the second, + the thread was holding pd_lock and found pd_busy not set, which would + mean that (2) was already not true. + b) ps_set_intr() and pi_schedule_claimed() can be called only from the + area. + c) pi_do_claimed() is called by pd.c only from the area. + d) ps_tq_int() can enter the area only when the thread is holding + ps_spinlock and ps_tq_active is 1 (due to Lemma 1). It means that + (3) was already not true. + e) do_pd_{read,write}* could be called only from the area. The only + case that needs consideration is call from pi_wake_up() and there + we would have to be called for the PIA that got ->claimed_cont + from pd.c. That could happen only if pi_do_claimed() had been + called from pd.c for that PIA, which happens only for PIA belonging + to pd.c. + f) pi_wake_up() can enter the area only when the thread is holding + pi_spinlock and ->claimed_cont is non-NULL for PIA belonging to + pd.c. It means that (4) was already not true. + +(2) can become not true only when pd_lock is released by the thread in question. + Indeed, pd_busy is reset only in the area and thread that resets + it is holding pd_lock. The only place within the area where we + release pd_lock is in pd_next_buf() (called from within the area). + But that code does not reset pd_busy, so pd_busy would have to be + 0 when pd_next_buf() had acquired pd_lock. If it become 0 while + we were acquiring the lock, (1) would be already false, since + the thread that had reset it would be in the area simulateously. + If it was 0 before we tried to acquire pd_lock, (2) would be + already false. + +For similar reasons, (3) can become not true only when ps_spinlock is released +by the thread in question. However, all such places within the area are right +after resetting ps_tq_active to 0. + +(4) is done the same way - all places where we release pi_spinlock within +the area are either after resetting ->claimed_cont to NULL while holding +pi_spinlock, or after not tocuhing ->claimed_cont since acquiring pi_spinlock +also in the area. The only place where ->claimed_cont is made non-NULL is +in the area, under pi_spinlock and we do not release it until after leaving +the area. + +QED. + + +Corollary 1: ps_tq_active can be killed. Indeed, the only place where we +check its value is in ps_set_intr() and if it had been non-zero at that +point, we would have violated either (2.1) (if it was set while ps_set_intr() +was acquiring ps_spinlock) or (2.3) (if it was set when we started to +acquire ps_spinlock). + +Corollary 2: ps_spinlock can be killed. Indeed, Lemma 1 and Lemma 2 show +that the only possible contention is between scheduling ps_tq followed by +immediate release of spinlock and beginning of execution of ps_tq on +another CPU. + +Corollary 3: assignment to pd_busy in do_pd_read_start() and do_pd_write_start() +can be killed. Indeed, we are not holding pd_lock and thus pd_busy is already +1 here. + +Corollary 4: in ps_tq_int() uses of con can be replaced with uses of +ps_continuation, since the latter is changed only from the area. +We don't need to reset it to NULL, since we are guaranteed that there +will be a call of ps_set_intr() before we look at ps_continuation again. +We can remove the check for ps_continuation being NULL for the same +reason - the value is guaranteed to be set by the last ps_set_intr() and +we never pass it NULL. Assignements in the beginning of ps_set_intr() +can be taken to callers as long as they remain within the area. |