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dd409de256
LKMM uses two relations for talking about UNLOCK+LOCK pairings:
1) po-unlock-lock-po, which handles UNLOCK+LOCK pairings
on the same CPU or immediate lock handovers on the same
lock variable
2) po;[UL];(co|po);[LKW];po, which handles UNLOCK+LOCK pairs
literally as described in rcupdate.h#L1002, i.e., even
after a sequence of handovers on the same lock variable.
The latter relation is used only once, to provide the guarantee
defined in rcupdate.h#L1002 by smp_mb__after_unlock_lock(), which
makes any UNLOCK+LOCK pair followed by the fence behave like a full
barrier.
This patch drops this use in favor of using po-unlock-lock-po
everywhere, which unifies the way the model talks about UNLOCK+LOCK
pairings. At first glance this seems to weaken the guarantee given
by LKMM: When considering a long sequence of lock handovers
such as below, where P0 hands the lock to P1, which hands it to P2,
which finally executes such an after_unlock_lock fence, the mb
relation currently links any stores in the critical section of P0
to instructions P2 executes after its fence, but not so after the
patch.
P0(int *x, int *y, spinlock_t *mylock)
{
spin_lock(mylock);
WRITE_ONCE(*x, 2);
spin_unlock(mylock);
WRITE_ONCE(*y, 1);
}
P1(int *y, int *z, spinlock_t *mylock)
{
int r0 = READ_ONCE(*y); // reads 1
spin_lock(mylock);
spin_unlock(mylock);
WRITE_ONCE(*z,1);
}
P2(int *z, int *d, spinlock_t *mylock)
{
int r1 = READ_ONCE(*z); // reads 1
spin_lock(mylock);
spin_unlock(mylock);
smp_mb__after_unlock_lock();
WRITE_ONCE(*d,1);
}
P3(int *x, int *d)
{
WRITE_ONCE(*d,2);
smp_mb();
WRITE_ONCE(*x,1);
}
exists (1:r0=1 /\ 2:r1=1 /\ x=2 /\ d=2)
Nevertheless, the ordering guarantee given in rcupdate.h is actually
not weakened. This is because the unlock operations along the
sequence of handovers are A-cumulative fences. They ensure that any
stores that propagate to the CPU performing the first unlock
operation in the sequence must also propagate to every CPU that
performs a subsequent lock operation in the sequence. Therefore any
such stores will also be ordered correctly by the fence even if only
the final handover is considered a full barrier.
Indeed this patch does not affect the behaviors allowed by LKMM at
all. The mb relation is used to define ordering through:
1) mb/.../ppo/hb, where the ordering is subsumed by hb+ where the
lock-release, rfe, and unlock-acquire orderings each provide hb
2) mb/strong-fence/cumul-fence/prop, where the rfe and A-cumulative
lock-release orderings simply add more fine-grained cumul-fence
edges to substitute a single strong-fence edge provided by a long
lock handover sequence
3) mb/strong-fence/pb and various similar uses in the definition of
data races, where as discussed above any long handover sequence
can be turned into a sequence of cumul-fence edges that provide
the same ordering.
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.