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Subject[patch v2] drivers/zram: Don't disable preemption in zcomp_stream_get/put()
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On Mon, 2016-10-17 at 16:24 +0200, Sebastian Andrzej Siewior wrote:
> On 2016-10-16 05:14:22 [+0200], Mike Galbraith wrote:
> >
> > In v4.7, the driver switched to percpu compression streams, disabling
> > preemption (get/put_cpu_ptr()). Use get/put_cpu_light() instead.
>
> I am not convinced that this will work. Nothing prevents
> zram_bvec_write() to be reentrant on the same CPU what I can tell from
> browsing over the code and since it uses zstrm->buffer for compression
> it can go wrong. Also I don't know if crypto's tfm handler can be used
> in parallel for any ops (it usually does not work for crypto).
>
> I suggest a local lock or a good reason why the this patch works.

(taking a break from hotplug squabble to pick on something easier:)

drivers/zram: Don't disable preemption in zcomp_stream_get/put()

In v4.7, the driver switched to percpu compression streams, disabling
preemption vai get/put_cpu_ptr(). Use a local lock instead for RT.
We also have to fix an RT lock order issue in zram_decompress_page()
such that zs_map_object() nests inside of zcomp_stream_put() as it
does in zram_bvec_write().

Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
---
drivers/block/zram/zcomp.c | 7 +++++--
drivers/block/zram/zram_drv.c | 11 +++++------
2 files changed, 10 insertions(+), 8 deletions(-)

--- a/drivers/block/zram/zcomp.c
+++ b/drivers/block/zram/zcomp.c
@@ -15,6 +15,7 @@
#include <linux/sched.h>
#include <linux/cpu.h>
#include <linux/crypto.h>
+#include <linux/locallock.h>

#include "zcomp.h"

@@ -116,14 +117,16 @@ ssize_t zcomp_available_show(const char
return sz;
}

+DEFINE_LOCAL_IRQ_LOCK(zram_stream_lock);
+
struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
{
- return *get_cpu_ptr(comp->stream);
+ return get_locked_var(zram_stream_lock, *comp->stream);
}

void zcomp_stream_put(struct zcomp *comp)
{
- put_cpu_ptr(comp->stream);
+ put_locked_var(zram_stream_lock, *comp->stream);
}

int zcomp_compress(struct zcomp_strm *zstrm,
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -566,6 +566,7 @@ static int zram_decompress_page(struct z
int ret = 0;
unsigned char *cmem;
struct zram_meta *meta = zram->meta;
+ struct zcomp_strm *zstrm;
unsigned long handle;
unsigned int size;

@@ -579,16 +580,14 @@ static int zram_decompress_page(struct z
return 0;
}

+ zstrm = zcomp_stream_get(zram->comp);
cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_RO);
- if (size == PAGE_SIZE) {
+ if (size == PAGE_SIZE)
copy_page(mem, cmem);
- } else {
- struct zcomp_strm *zstrm = zcomp_stream_get(zram->comp);
-
+ else
ret = zcomp_decompress(zstrm, cmem, size, mem);
- zcomp_stream_put(zram->comp);
- }
zs_unmap_object(meta->mem_pool, handle);
+ zcomp_stream_put(zram->comp);
zram_unlock_table(&meta->table[index]);

/* Should NEVER happen. Return bio error if it does. */

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