Commit 3c118547 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by David S. Miller
Browse files

u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates. 



On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit
architectures even on UP because the softirq (and the threaded IRQ handler) can
be preempted.

With the seqcount_t for synchronisation, a reader with higher priority can
preempt the writer and then spin endlessly in read_seqcount_begin() while the
writer can't make progress.

To avoid such a lock up on PREEMPT_RT the writer must disable preemption during
the update. There is no need to disable interrupts because no writer is using
this API in hard-IRQ context on PREEMPT_RT.

Disable preemption on 32bit-RT within the u64_stats write section.

Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent d147dd70

include/linux/u64_stats_sync.h

+28 −14
Original line number Diff line number Diff line
@@ -66,7 +66,7 @@ 
#include <linux/seqlock.h>



struct u64_stats_sync {

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	seqcount_t	seq;

#endif

};
@@ -125,7 +125,7 @@ static inline void u64_stats_inc(u64_stats_t *p) 
}

#endif



#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

#define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)

#else

static inline void u64_stats_init(struct u64_stats_sync *syncp)
@@ -135,15 +135,19 @@ static inline void u64_stats_init(struct u64_stats_sync *syncp) 


static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)

{

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	if (IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_disable();

	write_seqcount_begin(&syncp->seq);

#endif

}



static inline void u64_stats_update_end(struct u64_stats_sync *syncp)

{

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	write_seqcount_end(&syncp->seq);

	if (IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_enable();

#endif

}
@@ -152,7 +156,10 @@ u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp) 
{

	unsigned long flags = 0;



#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	if (IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_disable();

	else

		local_irq_save(flags);

	write_seqcount_begin(&syncp->seq);

#endif
@@ -163,15 +170,18 @@ static inline void 
u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,

				unsigned long flags)

{

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	write_seqcount_end(&syncp->seq);

	if (IS_ENABLED(CONFIG_PREEMPT_RT))

		preempt_enable();

	else

		local_irq_restore(flags);

#endif

}



static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)

{

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	return read_seqcount_begin(&syncp->seq);

#else

	return 0;
@@ -180,7 +190,7 @@ static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync * 


static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)

{

#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))

	preempt_disable();

#endif

	return __u64_stats_fetch_begin(syncp);
@@ -189,7 +199,7 @@ static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *sy 
static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,

					 unsigned int start)

{

#if BITS_PER_LONG==32 && defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))

	return read_seqcount_retry(&syncp->seq, start);

#else

	return false;
@@ -199,7 +209,7 @@ static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 
static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,

					 unsigned int start)

{

#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))

	preempt_enable();

#endif

	return __u64_stats_fetch_retry(syncp, start);
@@ -213,7 +223,9 @@ static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp, 
 */

static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)

{

#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)

	preempt_disable();

#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)

	local_irq_disable();

#endif

	return __u64_stats_fetch_begin(syncp);
@@ -222,7 +234,9 @@ static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync 
static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,

					     unsigned int start)

{

#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)

#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)

	preempt_enable();

#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)

	local_irq_enable();

#endif

	return __u64_stats_fetch_retry(syncp, start);