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authorPranith Kumar K <pkarampu@redhat.com>2013-12-11 09:33:53 +0530
committerAnand Avati <avati@redhat.com>2014-01-19 23:05:15 -0800
commit8d55c25f158921b508bff0e7f25158991913f922 (patch)
tree258500ed1f6b15d6f2f6848a5cbd883b8dc40ea2 /api/src/glfs-internal.h
parentc2b09dc87e0763dfdff1e93a1dc6cc4c05f091bf (diff)
syncop: Change return value of syncop
Problem: We found a day-1 bug when syncop_xxx() infra is used inside a synctask with compilation optimization (CFLAGS -O2). Detailed explanation of the Root cause: We found the bug in 'gf_defrag_migrate_data' in rebalance operation: Lets look at interesting parts of the function: int gf_defrag_migrate_data (xlator_t *this, gf_defrag_info_t *defrag, loc_t *loc, dict_t *migrate_data) { ..... code section - [ Loop ] while ((ret = syncop_readdirp (this, fd, 131072, offset, NULL, &entries)) != 0) { ..... code section - [ ERRNO-1 ] (errno of readdirp is stored in readdir_operrno by a thread) /* Need to keep track of ENOENT errno, that means, there is no need to send more readdirp() */ readdir_operrno = errno; ..... code section - [ SYNCOP-1 ] (syncop_getxattr is called by a thread) ret = syncop_getxattr (this, &entry_loc, &dict, GF_XATTR_LINKINFO_KEY); code section - [ ERRNO-2] (checking for failures of syncop_getxattr(). This may not always be executed in same thread which executed [SYNCOP-1]) if (ret < 0) { if (errno != ENODATA) { loglevel = GF_LOG_ERROR; defrag->total_failures += 1; ..... } the function above could be executed by thread(t1) till [SYNCOP-1] and code from [ERRNO-2] can be executed by a different thread(t2) because of the way syncop-infra schedules the tasks. when the code is compiled with -O2 optimization this is the assembly code that is generated: [ERRNO-1] 1165 readdir_operrno = errno; <<---- errno gets expanded as *(__errno_location()) 0x00007fd149d48b60 <+496>: callq 0x7fd149d410c0 <address@hidden> 0x00007fd149d48b72 <+514>: mov %rax,0x50(%rsp) <<------ Address returned by __errno_location() is stored in a special location in stack for later use. 0x00007fd149d48b77 <+519>: mov (%rax),%eax 0x00007fd149d48b79 <+521>: mov %eax,0x78(%rsp) .... [ERRNO-2] 1281 if (errno != ENODATA) { 0x00007fd149d492ae <+2366>: mov 0x50(%rsp),%rax <<----- Because it already stored the address returned by __errno_location(), it just dereferences the address to get the errno value. BUT THIS CODE NEED NOT BE EXECUTED BY SAME THREAD!!! 0x00007fd149d492b3 <+2371>: mov $0x9,%ebp 0x00007fd149d492b8 <+2376>: mov (%rax),%edi 0x00007fd149d492ba <+2378>: cmp $0x3d,%edi The problem is that __errno_location() value of t1 and t2 are different. So [ERRNO-2] ends up reading errno of t1 instead of errno of t2 even though t2 is executing [ERRNO-2] code section. When code is compiled without any optimization for [ERRNO-2]: 1281 if (errno != ENODATA) { 0x00007fd58e7a326f <+2237>: callq 0x7fd58e797300 <address@hidden><<--- As it is calling __errno_location() again it gets the location from t2 so it works as intended. 0x00007fd58e7a3274 <+2242>: mov (%rax),%eax 0x00007fd58e7a3276 <+2244>: cmp $0x3d,%eax 0x00007fd58e7a3279 <+2247>: je 0x7fd58e7a32a1 <gf_defrag_migrate_data+2287> Fix: Make syncop_xxx() return (-errno) value as the return value in case of errors and all the functions which make syncop_xxx() will need to use (-ret) to figure out the reason for failure in case of syncop_xxx() failures. Change-Id: I314d20dabe55d3e62ff66f3b4adb1cac2eaebb57 BUG: 1040356 Signed-off-by: Pranith Kumar K <pkarampu@redhat.com> Reviewed-on: http://review.gluster.org/6475 Tested-by: Gluster Build System <jenkins@build.gluster.com> Reviewed-by: Anand Avati <avati@redhat.com>
Diffstat (limited to 'api/src/glfs-internal.h')
-rw-r--r--api/src/glfs-internal.h20
1 files changed, 20 insertions, 0 deletions
diff --git a/api/src/glfs-internal.h b/api/src/glfs-internal.h
index ec1d5579df7..f04557323a0 100644
--- a/api/src/glfs-internal.h
+++ b/api/src/glfs-internal.h
@@ -18,6 +18,26 @@
#define DEFAULT_REVAL_COUNT 1
+/*
+ * syncop_xxx() calls are executed in two ways, one is inside a synctask where
+ * the executing function will do 'swapcontext' and the other is without
+ * synctask where the executing thread is made to wait using pthread_cond_wait.
+ * Executing thread may change when syncop_xxx() is executed inside a synctask.
+ * This leads to errno_location change i.e. errno may give errno of
+ * non-executing thread. So errno is not touched inside a synctask execution.
+ * All gfapi calls are executed using the second way of executing syncop_xxx()
+ * where the executing thread waits using pthread_cond_wait so it is ok to set
+ * errno in these cases. The following macro makes syncop_xxx() behave just
+ * like a system call, where -1 is returned and errno is set when a failure
+ * occurs.
+ */
+#define DECODE_SYNCOP_ERR(ret) do { \
+ if (ret < 0) { \
+ errno = -ret; \
+ ret = -1; \
+ } \
+ } while (0)
+
#define ESTALE_RETRY(ret,errno,reval,loc,label) do { \
if (ret == -1 && errno == ESTALE) { \
if (reval < DEFAULT_REVAL_COUNT) { \