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authorFlorin Coras <fcoras@cisco.com>2021-03-31 19:36:49 -0700
committerDave Barach <openvpp@barachs.net>2021-04-02 17:15:07 +0000
commit014dba38cb9579808a2134fd10a071e4f8c4e213 (patch)
tree8e03931df6e3c7d7d146574f76bd4cec7dad38ed /src/svm/fifo_segment.c
parent115012a0b1d9617c07bae5c82a19125b3c692258 (diff)
svm: lock-free fifo chunk list push and pop
This avoids chunk allocation/collection deadlocks if either of the sides crashes. Type: improvement Signed-off-by: Florin Coras <fcoras@cisco.com> Change-Id: I98619e6e035fa8688889ca34db2143c8898732df
Diffstat (limited to 'src/svm/fifo_segment.c')
-rw-r--r--src/svm/fifo_segment.c96
1 files changed, 56 insertions, 40 deletions
diff --git a/src/svm/fifo_segment.c b/src/svm/fifo_segment.c
index 00fb023f3cb..3e728eca71a 100644
--- a/src/svm/fifo_segment.c
+++ b/src/svm/fifo_segment.c
@@ -127,31 +127,22 @@ fsh_virtual_mem_update (fifo_segment_header_t * fsh, u32 slice_index,
fss->virtual_mem += n_bytes;
}
-static inline void
-fss_chunk_freelist_lock (fifo_segment_slice_t *fss)
+static inline int
+fss_chunk_fl_index_is_valid (fifo_segment_slice_t *fss, u32 fl_index)
{
- u32 free = 0;
- while (!clib_atomic_cmp_and_swap_acq_relax_n (&fss->chunk_lock, &free, 1, 0))
- {
- /* atomic load limits number of compare_exchange executions */
- while (clib_atomic_load_relax_n (&fss->chunk_lock))
- CLIB_PAUSE ();
- /* on failure, compare_exchange writes (*p)->lock into free */
- free = 0;
- }
+ return (fl_index < FS_CHUNK_VEC_LEN);
}
-static inline void
-fss_chunk_freelist_unlock (fifo_segment_slice_t *fss)
-{
- /* Make sure all reads/writes are complete before releasing the lock */
- clib_atomic_release (&fss->chunk_lock);
-}
+#define FS_CL_HEAD_MASK 0xFFFFFFFFFFFF
+#define FS_CL_HEAD_TMASK 0xFFFF000000000000
+#define FS_CL_HEAD_TINC (1ULL << 48)
-static inline int
-fss_chunk_fl_index_is_valid (fifo_segment_slice_t * fss, u32 fl_index)
+static svm_fifo_chunk_t *
+fss_chunk_free_list_head (fifo_segment_header_t *fsh,
+ fifo_segment_slice_t *fss, u32 fl_index)
{
- return (fl_index < FS_CHUNK_VEC_LEN);
+ fs_sptr_t headsp = clib_atomic_load_relax_n (&fss->free_chunks[fl_index]);
+ return fs_chunk_ptr (fsh, headsp & FS_CL_HEAD_MASK);
}
static void
@@ -159,10 +150,19 @@ fss_chunk_free_list_push (fifo_segment_header_t *fsh,
fifo_segment_slice_t *fss, u32 fl_index,
svm_fifo_chunk_t *c)
{
- fss_chunk_freelist_lock (fss);
- c->next = fss->free_chunks[fl_index];
- fss->free_chunks[fl_index] = fs_chunk_sptr (fsh, c);
- fss_chunk_freelist_unlock (fss);
+ fs_sptr_t old_head, new_head, csp;
+
+ csp = fs_chunk_sptr (fsh, c);
+ ASSERT (csp <= FS_CL_HEAD_MASK);
+ old_head = clib_atomic_load_relax_n (&fss->free_chunks[fl_index]);
+
+ do
+ {
+ c->next = old_head & FS_CL_HEAD_MASK;
+ new_head = csp + ((old_head + FS_CL_HEAD_TINC) & FS_CL_HEAD_TMASK);
+ }
+ while (!clib_atomic_cmp_and_swap_acq_relax (
+ &fss->free_chunks[fl_index], &old_head, &new_head, 1 /* weak */));
}
static void
@@ -170,32 +170,48 @@ fss_chunk_free_list_push_list (fifo_segment_header_t *fsh,
fifo_segment_slice_t *fss, u32 fl_index,
svm_fifo_chunk_t *head, svm_fifo_chunk_t *tail)
{
- fss_chunk_freelist_lock (fss);
- tail->next = fss->free_chunks[fl_index];
- fss->free_chunks[fl_index] = fs_chunk_sptr (fsh, head);
- fss_chunk_freelist_unlock (fss);
+ fs_sptr_t old_head, new_head, headsp;
+
+ headsp = fs_chunk_sptr (fsh, head);
+ ASSERT (headsp <= FS_CL_HEAD_MASK);
+ old_head = clib_atomic_load_relax_n (&fss->free_chunks[fl_index]);
+
+ do
+ {
+ tail->next = old_head & FS_CL_HEAD_MASK;
+ new_head = headsp + ((old_head + FS_CL_HEAD_TINC) & FS_CL_HEAD_TMASK);
+ }
+ while (!clib_atomic_cmp_and_swap_acq_relax (
+ &fss->free_chunks[fl_index], &old_head, &new_head, 1 /* weak */));
}
static svm_fifo_chunk_t *
fss_chunk_free_list_pop (fifo_segment_header_t *fsh, fifo_segment_slice_t *fss,
u32 fl_index)
{
+ fs_sptr_t old_head, new_head;
svm_fifo_chunk_t *c;
ASSERT (fss_chunk_fl_index_is_valid (fss, fl_index));
- fss_chunk_freelist_lock (fss);
+ old_head = clib_atomic_load_relax_n (&fss->free_chunks[fl_index]);
- if (!fss->free_chunks[fl_index])
+ /* Lock-free stacks are affected by ABA if a side allocates a chunk and
+ * shortly thereafter frees it. To circumvent that, reuse the upper bits
+ * of the head of the list shared pointer, i.e., offset to where the chunk
+ * is, as a tag. The tag is incremented with each push/pop operation and
+ * therefore collisions can only happen if an element is popped and pushed
+ * exactly after a complete wrap of the tag (16 bits). It's unlikely either
+ * of the sides will be descheduled for that long */
+ do
{
- fss_chunk_freelist_unlock (fss);
- return 0;
+ if (!(old_head & FS_CL_HEAD_MASK))
+ return 0;
+ c = fs_chunk_ptr (fsh, old_head & FS_CL_HEAD_MASK);
+ new_head = c->next + ((old_head + FS_CL_HEAD_TINC) & FS_CL_HEAD_TMASK);
}
-
- c = fs_chunk_ptr (fsh, fss->free_chunks[fl_index]);
- fss->free_chunks[fl_index] = c->next;
-
- fss_chunk_freelist_unlock (fss);
+ while (!clib_atomic_cmp_and_swap_acq_relax (
+ &fss->free_chunks[fl_index], &old_head, &new_head, 1 /* weak */));
return c;
}
@@ -1271,7 +1287,7 @@ fs_slice_num_free_chunks (fifo_segment_header_t *fsh,
{
for (i = 0; i < FS_CHUNK_VEC_LEN; i++)
{
- c = fs_chunk_ptr (fsh, fss->free_chunks[i]);
+ c = fss_chunk_free_list_head (fsh, fss, i);
if (c == 0)
continue;
@@ -1290,7 +1306,7 @@ fs_slice_num_free_chunks (fifo_segment_header_t *fsh,
if (fl_index >= FS_CHUNK_VEC_LEN)
return 0;
- c = fs_chunk_ptr (fsh, fss->free_chunks[fl_index]);
+ c = fss_chunk_free_list_head (fsh, fss, fl_index);
if (c == 0)
return 0;
@@ -1519,7 +1535,7 @@ format_fifo_segment (u8 * s, va_list * args)
fss = fsh_slice_get (fsh, slice_index);
for (i = 0; i < FS_CHUNK_VEC_LEN; i++)
{
- c = fs_chunk_ptr (fsh, fss->free_chunks[i]);
+ c = fss_chunk_free_list_head (fsh, fss, i);
if (c == 0 && fss->num_chunks[i] == 0)
continue;
count = 0;