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    SubjectRe: [PATCH 3/5] gpu/drm/ttm: Use mutex_trylock() to avoid deadlock inside shrinker functions.
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    Konrad Rzeszutek Wilk wrote:
    > Hmm..
    >
    > /home/konrad/linux/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c: In function ‘ttm_dma_pool_shrink_scan’:
    > /home/konrad/linux/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c:1015:2: error: invalid use of void expression
    > if (!mutex_lock(&_manager->lock))
    >
    > This is based on v3.15 with these patches.

    Wow! I didn't know that my gcc does not emit warning on such a mistake.
    Thank you for catching this.
    ----------
    >From 6e6774a87695408ef077cab576e76f7fa2cf4355 Mon Sep 17 00:00:00 2001
    From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Date: Wed, 11 Jun 2014 05:10:50 +0900
    Subject: [PATCH 3/5 (v2)] gpu/drm/ttm: Use mutex_trylock() to avoid deadlock inside shrinker functions.

    I can observe that RHEL7 environment stalls with 100% CPU usage when a
    certain type of memory pressure is given. While the shrinker functions
    are called by shrink_slab() before the OOM killer is triggered, the stall
    lasts for many minutes.

    One of reasons of this stall is that
    ttm_dma_pool_shrink_count()/ttm_dma_pool_shrink_scan() are called and
    are blocked at mutex_lock(&_manager->lock). GFP_KERNEL allocation with
    _manager->lock held causes someone (including kswapd) to deadlock when
    these functions are called due to memory pressure. This patch changes
    "mutex_lock();" to "if (!mutex_trylock()) return ...;" in order to
    avoid deadlock.

    Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: stable <stable@kernel.org> [3.3+]

    Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    ---
    drivers/gpu/drm/ttm/ttm_page_alloc_dma.c | 6 ++++--
    1 files changed, 4 insertions(+), 2 deletions(-)

    diff --git a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
    index d8e59f7..524cc1a 100644
    --- a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
    +++ b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
    @@ -1014,7 +1014,8 @@ ttm_dma_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
    if (list_empty(&_manager->pools))
    return SHRINK_STOP;

    - mutex_lock(&_manager->lock);
    + if (!mutex_trylock(&_manager->lock))
    + return SHRINK_STOP;
    if (!_manager->npools)
    goto out;
    pool_offset = ++start_pool % _manager->npools;
    @@ -1047,7 +1048,8 @@ ttm_dma_pool_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
    struct device_pools *p;
    unsigned long count = 0;

    - mutex_lock(&_manager->lock);
    + if (!mutex_trylock(&_manager->lock))
    + return 0;
    list_for_each_entry(p, &_manager->pools, pools)
    count += p->pool->npages_free;
    mutex_unlock(&_manager->lock);
    --
    1.7.1

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