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doc: update fuse interrupt documentation

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As Issue #1310 pointed out, for fops that
have interrupt handlers the fop handler
needs to pay attention to give the proper
FUSE response when it's interrupted.

This change extends the interrupt documentation
with guidelines regarding the FUSE response.

Also:
- improve wording
- add an 'Overview' section to explain the
  code flow before going in to the technical
  details

Change-Id: I852bfb717b1bde73f220878d6376429564413820
updates: #1374
Signed-off-by: Csaba Henk <csaba@redhat.com>
This commit is contained in:
Csaba Henk
2020-07-10 18:50:12 +02:00
committed by Amar Tumballi
parent 9334a8de8a
commit 1eae3379ae
1 changed files with 89 additions and 8 deletions
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97
doc/developer-guide/fuse-interrupt.md
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@@ -23,9 +23,10 @@ not exported to a header file).
```
enum fuse_interrupt_state {
INTERRUPT_NONE,
/* ... */
INTERRUPT_SQUELCHED,
INTERRUPT_HANDLED,
/* ... */
};
typedef enum fuse_interrupt_state fuse_interrupt_state_t;
struct fuse_interrupt_record;
@@ -62,8 +63,58 @@ dummy implementation only for demonstration purposes.) Flush is chosen
because a `FLUSH` interrupt is easy to trigger (see
*tests/features/interrupt.t*). Interrupt handling for flush is switched on
by `--fuse-flush-handle-interrupt` (a hidden glusterfs command line flag).
The flush interrupt handling code is guarded by the
`flush_handle_interrupt` Boolean member of `fuse_private_t`.
The implementation of flush interrupt is contained in the
`fuse_flush_interrupt_handler()` function and blocks guarded by the
```
if (priv->flush_handle_interrupt) { ...
```
conditional (where `priv` is a `*fuse_private_t`).
### Overview
"Regular" fuse fops and interrupt handlers interact via a list containing
interrupt records.
If a fop wishes to have its interrupts handled, it needs to set up an
interrupt record and insert it into the list; also when it's to finish
(ie. in its "cbk" stage) it needs to delete the record from the list.
If no interrupt happens, basically that's all to it - a list insertion
and deletion.
However, if an interrupt comes for the fop, the interrupt FUSE request
will carry the data identifying an ongoing fop (that is, its `unique`),
and based on that, the interrupt record will be looked up in the list, and
the specific interrupt handler (a member of the interrupt record) will be
called.
Usually the fop needs to share some data with the interrupt handler to
enable it to perform its task (also shared via the interrupt record).
The interrupt API offers two approaches to manage shared data:
- _Async or reference-counting strategy_: from the point on when the interrupt
record is inserted to the list, it's owned jointly by the regular fop and
the prospective interrupt handler. Both of them need to check before they
return if the other is still holding a reference; if not, then they are
responsible for reclaiming the shared data.
- _Sync or borrow strategy_: the interrupt handler is considered a borrower
of the shared data. The interrupt handler should not reclaim the shared
data. The fop will wait for the interrupt handler to finish (ie., the borrow
to be returned), then it has to reclaim the shared data.
The user of the interrupt API need to call the following functions to
instrument this control flow:
- `fuse_interrupt_record_insert()` in the fop to insert the interrupt record to
the list;
- `fuse_interrupt_finish_fop()`in the fop (cbk) and
- `fuse_interrupt_finish_interrupt()`in the interrupt handler
to perform needed synchronization at the end their tenure. The data management
strategies are implemented by the `fuse_interrupt_finish_*()` functions (which
have an argument to specify which strategy to use); these routines take care
of freeing the interrupt record itself, while the reclamation of the shared data
is left to the API user.
### Usage
@@ -75,12 +126,15 @@ steps:
call (directly or as async callback) `fuse_interrupt_finish_interrupt()`.
The `intstat` argument to `fuse_interrupt_finish_interrupt` should be
either `INTERRUPT_SQUELCHED` or `INTERRUPT_HANDLED`.
- `INTERRUPT_SQUELCHED` means that we choose not to handle the interrupt
- `INTERRUPT_SQUELCHED` means that the interrupt could not be delivered
and the fop is going on uninterrupted.
- `INTERRUPT_HANDLED` means that the interrupt was actually handled. In
this case the fop will be answered from interrupt context with errno
`EINTR` (that is, the fop should not send a response to the kernel).
(the enum `fuse_interrupt_state` includes further members, which are reserved
for internal use).
We return to the `sync` and `datap` arguments later.
- In the `fuse_<FOP>` function create an interrupt record using
`fuse_interrupt_record_new()`, passing the incoming `fuse_in_header` and
@@ -92,10 +146,10 @@ steps:
`fuse_interrupt_record_insert()`.
- In `fuse_<FOP>_cbk` call `fuse_interrupt_finish_fop()`.
- `fuse_interrupt_finish_fop()` returns a Boolean according to whether the
interrupt was handled. If it was, then the fuse request is already
interrupt was handled. If it was, then the FUSE request is already
answered and the stack gets destroyed in `fuse_interrupt_finish_fop` so
`fuse_<FOP>_cbk` can just return (zero). Otherwise follow the standard
cbk logic (answer the fuse request and destroy the stack -- these are
`fuse_<FOP>_cbk()` can just return (zero). Otherwise follow the standard
cbk logic (answer the FUSE request and destroy the stack -- these are
typically accomplished by `fuse_err_cbk()`).
- The last two argument of `fuse_interrupt_finish_fop()` and
`fuse_interrupt_finish_interrupt()` are `gf_boolean_t sync` and
@@ -124,7 +178,34 @@ steps:
then that pointer will be directed to the `data` member of the interrupt
record and it's up to the caller what it's doing with it.
- If `sync` is true, interrupt handler can use `datap = NULL`, and
fop handler will have `datap` set.
fop handler will have `datap` point to a valid pointer.
- If `sync` is false, and handlers pass a pointer to a pointer for
`datap`, they should check if the pointed pointer is NULL before
attempting to deal with the data.
### FUSE answer for the interrupted fop
The kernel acknowledges a successful interruption for a given FUSE request
if the filesystem daemon answers it with errno EINTR; upon that, the syscall
which induced the request will be abruptly terminated with an interrupt, rather
than returning a value.
In glusterfs, this can be arranged in two ways.
- If the interrupt handler wins the race for the interrupt record, ie.
`fuse_interrupt_finish_fop()` returns true to `fuse_<FOP>_cbk()`, then, as
said above, `fuse_<FOP>_cbk()` does not need to answer the FUSE request.
That's because then the interrupt handler will take care about answering
it (with errno EINTR).
- If `fuse_interrupt_finish_fop()` returns false to `fuse_<FOP>_cbk()`, then
this return value does not inform the fop handler whether there was an interrupt
or not. This return value occurs both when fop handler won the race for the
interrupt record against the interrupt handler, and when there was no interrupt
at all.
However, the internal logic of the fop handler might detect from other
circumstances that an interrupt was delivered. For example, the fop handler
might be sleeping, waiting for some data to arrive, so that a premature
wakeup (with no data present) occurs if the interrupt handler intervenes. In
such cases it's the responsibility of the fop handler to reply the FUSE
request with errro EINTR.