mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-03 17:40:04 +00:00
![Linus Torvalds](/assets/img/avatar_default.png)
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
128 lines
6.5 KiB
Text
128 lines
6.5 KiB
Text
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.
|