[PATCH 7/8] zram: Use local lock to protect per-CPU data

Sebastian Andrzej Siewior
2020-05-19T20:20:17Z

From: Mike Galbraith <umgwanakikbuti@gmail.com>

The zcomp driver uses per-CPU compression. The per-CPU data pointer is
acquired with get_cpu_ptr() which implicitly disables preemption.
It allocates memory inside the preempt disabled region which conflicts
with the PREEMPT_RT semantics.

Replace the implicit preemption control with an explicit local lock.
This allows RT kernels to substitute it with a real per CPU lock, which
serializes the access but keeps the code section preemptible. On non RT
kernels this maps to preempt_disable() as before, i.e. no functional
change.

[bigeasy: Use local_lock(), description, drop reordering]

Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
drivers/block/zram/zcomp.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
index 1a8564a79d8dc..32854d460b299 100644
--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -11,6 +11,7 @@
#include <linux/sched.h>
#include <linux/cpu.h>
#include <linux/crypto.h>
+#include <linux/locallock.h>

#include "zcomp.h"

@@ -111,14 +112,17 @@ ssize_t zcomp_available_show(const char *comp, char *buf)
return sz;
}

+static DEFINE_LOCAL_LOCK(zcomp_lock);
+
struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
{
- return *get_cpu_ptr(comp->stream);
+ local_lock(zcomp_lock);
+ return *this_cpu_ptr(comp->stream);
}

void zcomp_stream_put(struct zcomp *comp)
{
- put_cpu_ptr(comp->stream);
+ local_unlock(zcomp_lock);
}

int zcomp_compress(struct zcomp_strm *zstrm,
--
2.26.2