Messages in this thread | | | Date | Fri, 27 Sep 2013 22:58:56 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file |
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On 09/27/2013 02:09 PM, Tim Chen wrote: > On Fri, 2013-09-27 at 08:29 -0700, Paul E. McKenney wrote: >> On Wed, Sep 25, 2013 at 03:10:49PM -0700, Tim Chen wrote: >>> We will need the MCS lock code for doing optimistic spinning for rwsem. >>> Extracting the MCS code from mutex.c and put into its own file allow us >>> to reuse this code easily for rwsem. >>> >>> Signed-off-by: Tim Chen<tim.c.chen@linux.intel.com> >>> Signed-off-by: Davidlohr Bueso<davidlohr@hp.com> >>> --- >>> include/linux/mcslock.h | 58 +++++++++++++++++++++++++++++++++++++++++++++++ >>> kernel/mutex.c | 58 +++++----------------------------------------- >>> 2 files changed, 65 insertions(+), 51 deletions(-) >>> create mode 100644 include/linux/mcslock.h >>> >>> diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h >>> new file mode 100644 >>> index 0000000..20fd3f0 >>> --- /dev/null >>> +++ b/include/linux/mcslock.h >>> @@ -0,0 +1,58 @@ >>> +/* >>> + * MCS lock defines >>> + * >>> + * This file contains the main data structure and API definitions of MCS lock. >>> + */ >>> +#ifndef __LINUX_MCSLOCK_H >>> +#define __LINUX_MCSLOCK_H >>> + >>> +struct mcs_spin_node { >>> + struct mcs_spin_node *next; >>> + int locked; /* 1 if lock acquired */ >>> +}; >>> + >>> +/* >>> + * We don't inline mcs_spin_lock() so that perf can correctly account for the >>> + * time spent in this lock function. >>> + */ >>> +static noinline >>> +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) >>> +{ >>> + struct mcs_spin_node *prev; >>> + >>> + /* Init node */ >>> + node->locked = 0; >>> + node->next = NULL; >>> + >>> + prev = xchg(lock, node); >>> + if (likely(prev == NULL)) { >>> + /* Lock acquired */ >>> + node->locked = 1; >>> + return; >>> + } >>> + ACCESS_ONCE(prev->next) = node; >>> + smp_wmb(); >>> + /* Wait until the lock holder passes the lock down */ >>> + while (!ACCESS_ONCE(node->locked)) >>> + arch_mutex_cpu_relax(); >>> +} >>> + >>> +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node) >>> +{ >>> + struct mcs_spin_node *next = ACCESS_ONCE(node->next); >>> + >>> + if (likely(!next)) { >>> + /* >>> + * Release the lock by setting it to NULL >>> + */ >>> + if (cmpxchg(lock, node, NULL) == node) >>> + return; >>> + /* Wait until the next pointer is set */ >>> + while (!(next = ACCESS_ONCE(node->next))) >>> + arch_mutex_cpu_relax(); >>> + } >>> + ACCESS_ONCE(next->locked) = 1; >>> + smp_wmb(); >> Shouldn't the memory barrier precede the "ACCESS_ONCE(next->locked) = 1;"? >> Maybe in an "else" clause of the prior "if" statement, given that the >> cmpxchg() does it otherwise. >> >> Otherwise, in the case where the "if" conditionn is false, the critical >> section could bleed out past the unlock. > Yes, I agree with you that the smp_wmb should be moved before > ACCESS_ONCE to prevent critical section from bleeding. Copying Waiman > who is the original author of the mcs code to see if he has any comments > on things we may have missed. > > Tim
As a more general lock/unlock mechanism, I also agreed that we should move smp_wmb() before ACCESS_ONCE(). For the mutex case, it is used as a queuing mechanism rather than guarding critical section, so it doesn't really matter.
Regards, Longman
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