Speculation Example
One potential use for speculations is to highlight a particular code path. The following example shows the entire code path under the open(2) system call only when the open() fails:
Example 13–1 specopen.d: Code Flow for Failed open(2)
#!/usr/sbin/dtrace -Fs
syscall::open:entry,
syscall::open64:entry
{
/*
* The call to speculation() creates a new speculation. If this fails,
* dtrace(1M) will generate an error message indicating the reason for
* the failed speculation(), but subsequent speculative tracing will be
* silently discarded.
*/
self->spec = speculation();
speculate(self->spec);
/*
* Because this printf() follows the speculate(), it is being
* speculatively traced; it will only appear in the data buffer if the
* speculation is subsequently commited.
*/
printf("%s", stringof(copyinstr(arg0)));
}
fbt:::
/self->spec/
{
/*
* A speculate() with no other actions speculates the default action:
* tracing the EPID.
*/
speculate(self->spec);
}
syscall::open:return,
syscall::open64:return
/self->spec/
{
/*
* To balance the output with the -F option, we want to be sure that
* every entry has a matching return. Because we speculated the
* open entry above, we want to also speculate the open return.
* This is also a convenient time to trace the errno value.
*/
speculate(self->spec);
trace(errno);
}
syscall::open:return,
syscall::open64:return
/self->spec && errno != 0/
{
/*
* If errno is non-zero, we want to commit the speculation.
*/
commit(self->spec);
self->spec = 0;
}
syscall::open:return,
syscall::open64:return
/self->spec && errno == 0/
{
/*
* If errno is not set, we discard the speculation.
*/
discard(self->spec);
self->spec = 0;
}
Running the above script produces output similar to the following example:
# ./specopen.d dtrace: script './specopen.d' matched 24282 probes CPU FUNCTION 1 => open /var/ld/ld.config 1 -> open 1 -> copen 1 -> falloc 1 -> ufalloc 1 -> fd_find 1 -> mutex_owned 1 <- mutex_owned 1 <- fd_find 1 -> fd_reserve 1 -> mutex_owned 1 <- mutex_owned 1 -> mutex_owned 1 <- mutex_owned 1 <- fd_reserve 1 <- ufalloc 1 -> kmem_cache_alloc 1 -> kmem_cache_alloc_debug 1 -> verify_and_copy_pattern 1 <- verify_and_copy_pattern 1 -> file_cache_constructor 1 -> mutex_init 1 <- mutex_init 1 <- file_cache_constructor 1 -> tsc_gethrtime 1 <- tsc_gethrtime 1 -> getpcstack 1 <- getpcstack 1 -> kmem_log_enter 1 <- kmem_log_enter 1 <- kmem_cache_alloc_debug 1 <- kmem_cache_alloc 1 -> crhold 1 <- crhold 1 <- falloc 1 -> vn_openat 1 -> lookupnameat 1 -> copyinstr 1 <- copyinstr 1 -> lookuppnat 1 -> lookuppnvp 1 -> pn_fixslash 1 <- pn_fixslash 1 -> pn_getcomponent 1 <- pn_getcomponent 1 -> ufs_lookup 1 -> dnlc_lookup 1 -> bcmp 1 <- bcmp 1 <- dnlc_lookup 1 -> ufs_iaccess 1 -> crgetuid 1 <- crgetuid 1 -> groupmember 1 -> supgroupmember 1 <- supgroupmember 1 <- groupmember 1 <- ufs_iaccess 1 <- ufs_lookup 1 -> vn_rele 1 <- vn_rele 1 -> pn_getcomponent 1 <- pn_getcomponent 1 -> ufs_lookup 1 -> dnlc_lookup 1 -> bcmp 1 <- bcmp 1 <- dnlc_lookup 1 -> ufs_iaccess 1 -> crgetuid 1 <- crgetuid 1 <- ufs_iaccess 1 <- ufs_lookup 1 -> vn_rele 1 <- vn_rele 1 -> pn_getcomponent 1 <- pn_getcomponent 1 -> ufs_lookup 1 -> dnlc_lookup 1 -> bcmp 1 <- bcmp 1 <- dnlc_lookup 1 -> ufs_iaccess 1 -> crgetuid 1 <- crgetuid 1 <- ufs_iaccess 1 -> vn_rele 1 <- vn_rele 1 <- ufs_lookup 1 -> vn_rele 1 <- vn_rele 1 <- lookuppnvp 1 <- lookuppnat 1 <- lookupnameat 1 <- vn_openat 1 -> setf 1 -> fd_reserve 1 -> mutex_owned 1 <- mutex_owned 1 -> mutex_owned 1 <- mutex_owned 1 <- fd_reserve 1 -> cv_broadcast 1 <- cv_broadcast 1 <- setf 1 -> unfalloc 1 -> mutex_owned 1 <- mutex_owned 1 -> crfree 1 <- crfree 1 -> kmem_cache_free 1 -> kmem_cache_free_debug 1 -> kmem_log_enter 1 <- kmem_log_enter 1 -> tsc_gethrtime 1 <- tsc_gethrtime 1 -> getpcstack 1 <- getpcstack 1 -> kmem_log_enter 1 <- kmem_log_enter 1 -> file_cache_destructor 1 -> mutex_destroy 1 <- mutex_destroy 1 <- file_cache_destructor 1 -> copy_pattern 1 <- copy_pattern 1 <- kmem_cache_free_debug 1 <- kmem_cache_free 1 <- unfalloc 1 -> set_errno 1 <- set_errno 1 <- copen 1 <- open 1 <= open 2 |
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