Hello, On 2014-02-12 17:33, Russell King - ARM Linux wrote: > On Wed, Feb 12, 2014 at 04:40:50PM +0100, Marek Szyprowski wrote: > > Hello, > > > > On 2014-02-11 19:35, Russell King - ARM Linux wrote: > >> The cubox-i4 just hit a new lockdep problem - not quite sure what to > >> make of this - it looks like an interaction between quite a lot of > >> locks - I suspect more than the lockdep code is reporting in its > >> "Possible unsafe locking scenario" report. > >> > >> I'm hoping I've sent this to appropriate people... if anyone thinks > >> this needs to go to someone else, please forward it. Thanks. > > > > From the attached log it looks like an issue (AB-BA deadlock) between > > device mutex (&dev->struct_mutex) and mm semaphore (&mm->mmap_sem). > > Similar issue has been discussed quite a long time ago in v4l2 > > subsystem: > > I think there's more locks involved than just those two. > > > https://www.mail-archive.com/linux-media at vger.kernel.org/msg38599.html > > http://www.spinics.net/lists/linux-media/msg40225.html > > > > Solving it probably requires some changes in DRM core. I see no direct > > relation between this issue and CMA itself. > > I don't think so - the locking in DRM is pretty sane. Let's take a > look: > > >> the existing dependency chain (in reverse order) is: > >> -> #5 (&dev->struct_mutex){+.+...}: > >> [<c0066f04>] __lock_acquire+0x151c/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c0698180>] mutex_lock_nested+0x5c/0x3ac > >> [<c0350c30>] drm_gem_mmap+0x40/0xdc > >> [<c03671d8>] drm_gem_cma_mmap+0x14/0x2c > >> [<c00ef4f4>] mmap_region+0x3ac/0x59c > >> [<c00ef9ac>] do_mmap_pgoff+0x2c8/0x370 > >> [<c00dd730>] vm_mmap_pgoff+0x6c/0x9c > >> [<c00ee1fc>] SyS_mmap_pgoff+0x54/0x98 > >> [<c000e6e0>] ret_fast_syscall+0x0/0x48 > > vm_mmap_pgoff() takes mm->mmap_sem before calling do_mmap_pgoff(). > So, this results in the following locking order: > > mm->mmap_sem > dev->struct_mutex > > >> -> #4 (&mm->mmap_sem){++++++}: > >> [<c0066f04>] __lock_acquire+0x151c/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c00e6c5c>] might_fault+0x6c/0x94 > >> [<c0335440>] con_set_unimap+0x158/0x27c > >> [<c032f800>] vt_ioctl+0x1298/0x1388 > >> [<c0323f44>] tty_ioctl+0x168/0xbf4 > >> [<c0115fac>] do_vfs_ioctl+0x84/0x664 > >> [<c01165d0>] SyS_ioctl+0x44/0x64 > >> [<c000e6e0>] ret_fast_syscall+0x0/0x48 > > vt_ioctl() takes the console lock, so this results in: > > console_lock > mm->mmap_sem > > >> -> #3 (console_lock){+.+.+.}: > >> [<c0066f04>] __lock_acquire+0x151c/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c006edcc>] console_lock+0x60/0x74 > >> [<c006f7b8>] console_cpu_notify+0x28/0x34 > >> [<c004904c>] notifier_call_chain+0x4c/0x8c > >> [<c004916c>] __raw_notifier_call_chain+0x1c/0x24 > >> [<c0024124>] __cpu_notify+0x34/0x50 > >> [<c002424c>] cpu_notify_nofail+0x18/0x24 > >> [<c068e168>] _cpu_down+0x100/0x244 > >> [<c068e2dc>] cpu_down+0x30/0x44 > >> [<c036ef8c>] cpu_subsys_offline+0x14/0x18 > >> [<c036af28>] device_offline+0x94/0xbc > >> [<c036b030>] online_store+0x4c/0x74 > >> [<c0368d3c>] dev_attr_store+0x20/0x2c > >> [<c016b2e0>] sysfs_kf_write+0x54/0x58 > >> [<c016eaa4>] kernfs_fop_write+0xc4/0x160 > >> [<c0105a54>] vfs_write+0xbc/0x184 > >> [<c0105dfc>] SyS_write+0x48/0x70 > >> [<c000e6e0>] ret_fast_syscall+0x0/0x48 > > cpu_down() takes cpu_hotplug.lock, so here we have: > > cpu_hotplug.lock > console_lock > > >> -> #2 (cpu_hotplug.lock){+.+.+.}: > >> [<c0066f04>] __lock_acquire+0x151c/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c0698180>] mutex_lock_nested+0x5c/0x3ac > >> [<c0024218>] get_online_cpus+0x3c/0x58 > >> [<c00d0ab0>] lru_add_drain_all+0x24/0x190 > >> [<c0101d3c>] migrate_prep+0x10/0x18 > >> [<c00cba04>] alloc_contig_range+0xf4/0x30c > >> [<c0371588>] dma_alloc_from_contiguous+0x7c/0x130 > >> [<c0018ef8>] __alloc_from_contiguous+0x38/0x12c > >> [<c0908694>] atomic_pool_init+0x74/0x128 > >> [<c0008850>] do_one_initcall+0x3c/0x164 > >> [<c0903c98>] kernel_init_freeable+0x104/0x1d0 > >> [<c068de54>] kernel_init+0x10/0xec > >> [<c000e7a8>] ret_from_fork+0x14/0x2c > > dma_alloc_from_contiguous takes the cma_mutex, so here we end up with: > > cma_mutex > cpu_hotplug.lock > > >> -> #1 (lock){+.+...}: > >> [<c0066f04>] __lock_acquire+0x151c/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c0698180>] mutex_lock_nested+0x5c/0x3ac > >> [<c00d0aa8>] lru_add_drain_all+0x1c/0x190 > >> [<c0101d3c>] migrate_prep+0x10/0x18 > >> [<c00cba04>] alloc_contig_range+0xf4/0x30c > >> [<c0371588>] dma_alloc_from_contiguous+0x7c/0x130 > >> [<c0018ef8>] __alloc_from_contiguous+0x38/0x12c > >> [<c0908694>] atomic_pool_init+0x74/0x128 > >> [<c0008850>] do_one_initcall+0x3c/0x164 > >> [<c0903c98>] kernel_init_freeable+0x104/0x1d0 > >> [<c068de54>] kernel_init+0x10/0xec > >> [<c000e7a8>] ret_from_fork+0x14/0x2c > > Ditto - here we have: > > cma_mutex > lock > > where "lock" is nicely named... this is a lock inside lru_add_drain_all() > and under this lock, we call get_online_cpus() and put_online_cpus(). > get_online_cpus() takes cpu_hotplug.lock, so here we also have: > > cma_mutex > lock > cpu_hotplug.lock > > >> -> #0 (cma_mutex){+.+.+.}: > >> [<c0690850>] print_circular_bug+0x70/0x2f0 > >> [<c0066f68>] __lock_acquire+0x1580/0x1ca0 > >> [<c0067c28>] lock_acquire+0xa0/0x130 > >> [<c0698180>] mutex_lock_nested+0x5c/0x3ac > >> [<c03716f4>] dma_release_from_contiguous+0xb8/0xf8 > >> [<c00197a4>] __arm_dma_free.isra.11+0x194/0x218 > >> [<c0019868>] arm_dma_free+0x1c/0x24 > >> [<c0366e34>] drm_gem_cma_free_object+0x68/0xb8 > >> [<c0351194>] drm_gem_object_free+0x30/0x38 > >> [<c0351318>] drm_gem_object_handle_unreference_unlocked+0x108/0x148 > >> [<c0351498>] drm_gem_handle_delete+0xb0/0x10c > >> [<c0351508>] drm_gem_dumb_destroy+0x14/0x18 > >> [<c035e838>] drm_mode_destroy_dumb_ioctl+0x34/0x40 > >> [<c034f918>] drm_ioctl+0x3f4/0x498 > >> [<c0115fac>] do_vfs_ioctl+0x84/0x664 > >> [<c01165d0>] SyS_ioctl+0x44/0x64 > >> [<c000e6e0>] ret_fast_syscall+0x0/0x48 > > drm_gem_object_unreference_unlocked takes dev->struct_mutex, so: > > dev->struct_mutex > cma_mutex > > > So, the full locking dependency tree is this: > > CPU0 CPU1 CPU2 CPU3 CPU4 > dev->struct_mutex (from #0) > mm->mmap_sem > dev->struct_mutex (from #5) > console_lock (from #4) > mm->mmap_sem > cpu_hotplug.lock (from #3) > console_lock > cma_mutex (from > #2, but also from #1) > cpu_hotplug.lock > cma_mutex > > Which is pretty sick - and I don't think that blaming this solely on V4L2 > nor DRM is particularly fair. I believe the onus is on every author of > one of those locks involved in that chain needs to re-analyse whether > their locking is sane. > > For instance, what is cma_mutex protecting? Is it protecting the CMA > bitmap?
This lock is protecting CMA bitmap and also serializes all CMA allocations. It is required by memory management core to serialize all calls to alloc_contig_range() (otherwise page block's migrate types might get overwritten). I don't see any other obvious solution for serializing alloc_contig_range() calls. > What if we did these changes: > > struct page *dma_alloc_from_contiguous(struct device *dev, int count, > unsigned int align) > { > ... > mutex_lock(&cma_mutex); > ... > for (;;) { > pageno = bitmap_find_next_zero_area(cma->bitmap, cma->count, > start, count, mask); > if (pageno >= cma->count) > break; > > pfn = cma->base_pfn + pageno; > + bitmap_set(cma->bitmap, pageno, count); > + mutex_unlock(&cma_mutex); > ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA); > + mutex_lock(&cma_mutex); > if (ret == 0) { > - bitmap_set(cma->bitmap, pageno, count); > page = pfn_to_page(pfn); > break; > - } else if (ret != -EBUSY) { > + } > + bitmap_clear(cma->bitmap, pageno, count); > + if (ret != -EBUSY) { > break; > } > ... > mutex_unlock(&cma_mutex); > pr_debug("%s(): returned %p\n", __func__, page); > return page; > } > > > bool dma_release_from_contiguous(struct device *dev, struct page *pages, > int count) > { > ... > + free_contig_range(pfn, count); > mutex_lock(&cma_mutex); > bitmap_clear(cma->bitmap, pfn - cma->base_pfn, count); > - free_contig_range(pfn, count); > mutex_unlock(&cma_mutex); > ... > } > > which avoids the dependency between cma_mutex and cpu_hotplug.lock ? This will not work correctly if there will be 2 concurrent calls to alloc_contig_range(), which will touch the same memory page blocks. Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland