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authorWaiman Long <Waiman.Long@hp.com>2015-06-19 11:50:01 -0400
committerIngo Molnar <mingo@kernel.org>2015-07-06 14:11:28 +0200
commit0e06e5be70d392aa842c1455ec2d0baf62aeed48 (patch)
treec4974970b68768690e0516927b51a7b3c8800014 /include/asm-generic
parentf7d71f2052555ae57b47322f2c2f6c29ff2438ae (diff)
locking/qrwlock: Better optimization for interrupt context readers
The qrwlock is fair in the process context, but becoming unfair when in the interrupt context to support use cases like the tasklist_lock. The current code isn't that well-documented on what happens when in the interrupt context. The rspin_until_writer_unlock() will only spin if the writer has gotten the lock. If the writer is still in the waiting state, the increment in the reader count will cause the writer to remain in the waiting state and the new interrupt context reader will get the lock and return immediately. The current code, however, does an additional read of the lock value which is not necessary as the information has already been there in the fast path. This may sometime cause an additional cacheline transfer when the lock is highly contended. This patch passes the lock value information gotten in the fast path to the slow path to eliminate the additional read. It also documents the action for the interrupt context readers more clearly. Signed-off-by: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Will Deacon <will.deacon@arm.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Douglas Hatch <doug.hatch@hp.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Scott J Norton <scott.norton@hp.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1434729002-57724-3-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'include/asm-generic')
-rw-r--r--include/asm-generic/qrwlock.h4
1 files changed, 2 insertions, 2 deletions
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 55e3ee1d2415..deb9e8b0eb9e 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -36,7 +36,7 @@
/*
* External function declarations
*/
-extern void queued_read_lock_slowpath(struct qrwlock *lock);
+extern void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts);
extern void queued_write_lock_slowpath(struct qrwlock *lock);
/**
@@ -105,7 +105,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
return;
/* The slowpath will decrement the reader count, if necessary. */
- queued_read_lock_slowpath(lock);
+ queued_read_lock_slowpath(lock, cnts);
}
/**

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