Lockdep noticed that there is chance for a deadlock if we have
concurrent mmap, concurrent read, and the addition/removal of a
callback.
As nicely explained by Boqun:
"
Lockdep warned about the above sequences because rw_semaphore is a fair
read-write lock, and the following can cause a deadlock:
TASK 1 TASK 2 TASK 3
====== ====== ======
down_write(mmap_lock);
down_read(vmcore_cb_rwsem)
down_write(vmcore_cb_rwsem); // blocked
down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
down_read(mmap_lock); // blocked
IOW, a reader can block another read if there is a writer queued by the
second reader and the lock is fair.
"
To fix, convert to srcu to make this deadlock impossible. We need srcu as
our callbacks can sleep. With this change, I cannot trigger any lockdep
warnings.
[ 6.386519] ======================================================
[ 6.387203] WARNING: possible circular locking dependency detected
[ 6.387965] 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1 Not tainted
[ 6.388899] ------------------------------------------------------
[ 6.389657] makedumpfile/542 is trying to acquire lock:
[ 6.390308] ffffffff832d2eb8 (vmcore_cb_rwsem){.+.+}-{3:3}, at: mmap_vmcore+0x340/0x580
[ 6.391290]
[ 6.391290] but task is already holding lock:
[ 6.391978] ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150
[ 6.392898]
[ 6.392898] which lock already depends on the new lock.
[ 6.392898]
[ 6.393866]
[ 6.393866] the existing dependency chain (in reverse order) is:
[ 6.394762]
[ 6.394762] -> #1 (&mm->mmap_lock#2){++++}-{3:3}:
[ 6.395530] lock_acquire+0xc3/0x1a0
[ 6.396047] __might_fault+0x4e/0x70
[ 6.396562] _copy_to_user+0x1f/0x90
[ 6.397093] __copy_oldmem_page+0x72/0xc0
[ 6.397663] read_from_oldmem+0x77/0x1e0
[ 6.398229] read_vmcore+0x2c2/0x310
[ 6.398742] proc_reg_read+0x47/0xa0
[ 6.399265] vfs_read+0x101/0x340
[ 6.399751] __x64_sys_pread64+0x5d/0xa0
[ 6.400314] do_syscall_64+0x43/0x90
[ 6.400778] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 6.401390]
[ 6.401390] -> #0 (vmcore_cb_rwsem){.+.+}-{3:3}:
[ 6.402063] validate_chain+0x9f4/0x2670
[ 6.402560] __lock_acquire+0x8f7/0xbc0
[ 6.403054] lock_acquire+0xc3/0x1a0
[ 6.403509] down_read+0x4a/0x140
[ 6.403948] mmap_vmcore+0x340/0x580
[ 6.404403] proc_reg_mmap+0x3e/0x90
[ 6.404866] mmap_region+0x504/0x880
[ 6.405322] do_mmap+0x38a/0x520
[ 6.405744] vm_mmap_pgoff+0xc1/0x150
[ 6.406258] ksys_mmap_pgoff+0x178/0x200
[ 6.406823] do_syscall_64+0x43/0x90
[ 6.407339] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 6.407975]
[ 6.407975] other info that might help us debug this:
[ 6.407975]
[ 6.408945] Possible unsafe locking scenario:
[ 6.408945]
[ 6.409684] CPU0 CPU1
[ 6.410196] ---- ----
[ 6.410703] lock(&mm->mmap_lock#2);
[ 6.411121] lock(vmcore_cb_rwsem);
[ 6.411792] lock(&mm->mmap_lock#2);
[ 6.412465] lock(vmcore_cb_rwsem);
[ 6.412873]
[ 6.412873] *** DEADLOCK ***
[ 6.412873]
[ 6.413522] 1 lock held by makedumpfile/542:
[ 6.414006] #0: ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150
[ 6.414944]
[ 6.414944] stack backtrace:
[ 6.415432] CPU: 0 PID: 542 Comm: makedumpfile Not tainted 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1
[ 6.416581] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[ 6.417272] Call Trace:
[ 6.417593] <TASK>
[ 6.417882] dump_stack_lvl+0x5d/0x78
[ 6.418346] print_circular_bug+0x5d7/0x5f0
[ 6.418821] ? stack_trace_save+0x3a/0x50
[ 6.419273] ? save_trace+0x3d/0x330
[ 6.419681] check_noncircular+0xd1/0xe0
[ 6.420217] validate_chain+0x9f4/0x2670
[ 6.420715] ? __lock_acquire+0x8f7/0xbc0
[ 6.421234] ? __lock_acquire+0x8f7/0xbc0
[ 6.421685] __lock_acquire+0x8f7/0xbc0
[ 6.422127] lock_acquire+0xc3/0x1a0
[ 6.422535] ? mmap_vmcore+0x340/0x580
[ 6.422965] ? lock_is_held_type+0xe2/0x140
[ 6.423432] ? mmap_vmcore+0x340/0x580
[ 6.423893] down_read+0x4a/0x140
[ 6.424321] ? mmap_vmcore+0x340/0x580
[ 6.424800] mmap_vmcore+0x340/0x580
[ 6.425237] ? vm_area_alloc+0x1c/0x60
[ 6.425661] ? trace_kmem_cache_alloc+0x30/0xe0
[ 6.426174] ? kmem_cache_alloc+0x1e0/0x2f0
[ 6.426641] proc_reg_mmap+0x3e/0x90
[ 6.427052] mmap_region+0x504/0x880
[ 6.427462] do_mmap+0x38a/0x520
[ 6.427842] vm_mmap_pgoff+0xc1/0x150
[ 6.428260] ksys_mmap_pgoff+0x178/0x200
[ 6.428701] do_syscall_64+0x43/0x90
[ 6.429126] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 6.429745] RIP: 0033:0x7fc7359b8fc7
[ 6.430157] Code: 00 00 00 89 ef e8 69 b3 ff ff eb e4 e8 c2 64 01 00 66 90 f3 0f 1e fa 41 89 ca 41 f7 c1 ff 0f 00 00 75 10 b8 09 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 21 c3 48 8b 05 21 7e 0e 00 64 c7 00 16 00 00
[ 6.432147] RSP: 002b:00007fff35b4c208 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
[ 6.432970] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fc7359b8fc7
[ 6.433746] RDX: 0000000000000001 RSI: 0000000000400000 RDI: 0000000000000000
[ 6.434529] RBP: 000055a1125ecf10 R08: 0000000000000003 R09: 0000000000002000
[ 6.435310] R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000002000
[ 6.436093] R13: 0000000000400000 R14: 000055a1124269e2 R15: 0000000000000000
[ 6.436887] </TASK>
Reported-by: Baoquan He <bhe@xxxxxxxxxx>
Fixes: cc5f2704c934 ("proc/vmcore: convert oldmem_pfn_is_ram callback to more generic vmcore callbacks")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
Cc: Dave Young <dyoung@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
Was: [PATCH v1] proc/vmcore: fix false positive lockdep warning
v1 -> v2:
- Adjust subject/description
- Add Fixes:
---
fs/proc/vmcore.c | 41 ++++++++++++++++++++++-------------------
1 file changed, 22 insertions(+), 19 deletions(-)
diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 702754dd1daf..edeb01dfe05d 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -62,7 +62,8 @@ core_param(novmcoredd, vmcoredd_disabled, bool, 0);
/* Device Dump Size */
static size_t vmcoredd_orig_sz;
-static DECLARE_RWSEM(vmcore_cb_rwsem);
+static DEFINE_SPINLOCK(vmcore_cb_lock);
+DEFINE_STATIC_SRCU(vmcore_cb_srcu);
/* List of registered vmcore callbacks. */
static LIST_HEAD(vmcore_cb_list);
/* Whether the vmcore has been opened once. */
@@ -70,8 +71,8 @@ static bool vmcore_opened;
void register_vmcore_cb(struct vmcore_cb *cb)
{
- down_write(&vmcore_cb_rwsem);
INIT_LIST_HEAD(&cb->next);
+ spin_lock(&vmcore_cb_lock);
list_add_tail(&cb->next, &vmcore_cb_list);
/*
* Registering a vmcore callback after the vmcore was opened is
@@ -79,14 +80,14 @@ void register_vmcore_cb(struct vmcore_cb *cb)
*/
if (vmcore_opened)
pr_warn_once("Unexpected vmcore callback registration\n");
- up_write(&vmcore_cb_rwsem);
+ spin_unlock(&vmcore_cb_lock);
}
EXPORT_SYMBOL_GPL(register_vmcore_cb);
void unregister_vmcore_cb(struct vmcore_cb *cb)
{
- down_write(&vmcore_cb_rwsem);
- list_del(&cb->next);
+ spin_lock(&vmcore_cb_lock);
+ list_del_rcu(&cb->next);
/*
* Unregistering a vmcore callback after the vmcore was opened is
* very unusual (e.g., forced driver removal), but we cannot stop
@@ -94,7 +95,9 @@ void unregister_vmcore_cb(struct vmcore_cb *cb)
*/
if (vmcore_opened)
pr_warn_once("Unexpected vmcore callback unregistration\n");
- up_write(&vmcore_cb_rwsem);
+ spin_unlock(&vmcore_cb_lock);
+
+ synchronize_srcu(&vmcore_cb_srcu);
}
EXPORT_SYMBOL_GPL(unregister_vmcore_cb);
@@ -103,9 +106,8 @@ static bool pfn_is_ram(unsigned long pfn)
struct vmcore_cb *cb;
bool ret = true;
- lockdep_assert_held_read(&vmcore_cb_rwsem);
-
- list_for_each_entry(cb, &vmcore_cb_list, next) {
+ list_for_each_entry_srcu(cb, &vmcore_cb_list, next,
+ srcu_read_lock_held(&vmcore_cb_srcu)) {
if (unlikely(!cb->pfn_is_ram))
continue;
ret = cb->pfn_is_ram(cb, pfn);
@@ -118,9 +120,9 @@ static bool pfn_is_ram(unsigned long pfn)
static int open_vmcore(struct inode *inode, struct file *file)
{
- down_read(&vmcore_cb_rwsem);
+ spin_lock(&vmcore_cb_lock);
vmcore_opened = true;
- up_read(&vmcore_cb_rwsem);
+ spin_unlock(&vmcore_cb_lock);
return 0;
}
@@ -133,6 +135,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
unsigned long pfn, offset;
size_t nr_bytes;
ssize_t read = 0, tmp;
+ int idx;
if (!count)
return 0;
@@ -140,7 +143,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
offset = (unsigned long)(*ppos % PAGE_SIZE);
pfn = (unsigned long)(*ppos / PAGE_SIZE);
- down_read(&vmcore_cb_rwsem);
+ idx = srcu_read_lock(&vmcore_cb_srcu);
do {
if (count > (PAGE_SIZE - offset))
nr_bytes = PAGE_SIZE - offset;
@@ -165,7 +168,7 @@ ssize_t read_from_oldmem(char *buf, size_t count,
offset, userbuf);
}
if (tmp < 0) {
- up_read(&vmcore_cb_rwsem);
+ srcu_read_unlock(&vmcore_cb_srcu, idx);
return tmp;
}
@@ -176,8 +179,8 @@ ssize_t read_from_oldmem(char *buf, size_t count,
++pfn;
offset = 0;
} while (count);
+ srcu_read_unlock(&vmcore_cb_srcu, idx);
- up_read(&vmcore_cb_rwsem);
return read;
}
@@ -568,18 +571,18 @@ static int vmcore_remap_oldmem_pfn(struct vm_area_struct *vma,
unsigned long from, unsigned long pfn,
unsigned long size, pgprot_t prot)
{
- int ret;
+ int ret, idx;
/*
- * Check if oldmem_pfn_is_ram was registered to avoid
- * looping over all pages without a reason.
+ * Check if a callback was registered to avoid looping over all
+ * pages without a reason.
*/
- down_read(&vmcore_cb_rwsem);
+ idx = srcu_read_lock(&vmcore_cb_srcu);
if (!list_empty(&vmcore_cb_list))
ret = remap_oldmem_pfn_checked(vma, from, pfn, size, prot);
else
ret = remap_oldmem_pfn_range(vma, from, pfn, size, prot);
- up_read(&vmcore_cb_rwsem);
+ srcu_read_unlock(&vmcore_cb_srcu, idx);
return ret;
}
--
2.34.1
